test(page-templates): cover duplicatePage pageEmbed remap + reference sync (#32)

Extract the per-node pageEmbed remap decision into a shared pure helper
(remapPageEmbedSourceId) and use it BOTH in PageService.duplicatePage and the
JSON walker, so the test guards the real production path (not a mirror that
could drift). Behavior is identical: source in the copied set -> new copy id;
otherwise keep the original. Add jest coverage (16 tests): the remap helper
(in-set/out-of-set/null/nested), syncPageTemplateReferences toDelete (stale refs
removed with the right workspaceId), and insertTemplateReferencesForPages
multi-workspace grouping/filtering.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/core/page/services/page.service.ts

@@ -62,6 +62,7 @@ import { markdownToHtml } from '@docmost/editor-ext';
 import { WatcherService } from '../../watcher/watcher.service';
 import { sql } from 'kysely';
 import { TransclusionService } from '../transclusion/transclusion.service';
+import { remapPageEmbedSourceId } from '../transclusion/utils/transclusion-prosemirror.util';
 import { AuthProvenanceData } from '../../../common/decorators/auth-provenance.decorator';
 
 @Injectable()
@@ -713,12 +714,11 @@ export class PageService {
           // source page is also part of the copied set, point at its new copy;
           // otherwise leave it pointing at the original (live embed of original).
           if (node.type.name === 'pageEmbed') {
-            const sourcePageId = node.attrs.sourcePageId;
-            if (sourcePageId && pageMap.has(sourcePageId)) {
-              const mappedPage = pageMap.get(sourcePageId);
-              // @ts-expect-error ProseMirror Attrs is read-only typed; reassigning sourcePageId to the duplicated page copy is intentional here
-              node.attrs.sourcePageId = mappedPage.newPageId;
-            }
+            // @ts-expect-error ProseMirror Attrs is read-only typed; intentional remap to the duplicated copy
+            node.attrs.sourcePageId = remapPageEmbedSourceId(
+              node.attrs.sourcePageId,
+              (id) => pageMap.get(id)?.newPageId,
+            );
           }
 
           // Update internal page links in link marks

apps/server/src/core/page/transclusion/spec/page-embed-remap.util.spec.ts

@@ -0,0 +1,200 @@
+import {
+  remapPageEmbedSourceId,
+  remapPageEmbedSourceIds,
+} from '../utils/transclusion-prosemirror.util';
+
+/**
+ * Unit tests for the `pageEmbed` remap used by `PageService.duplicatePage`:
+ *
+ *   - source page within the copied set  -> rewrite to the COPY's new id
+ *   - source page NOT in the copied set   -> keep the ORIGINAL id (live embed)
+ *
+ * `remapPageEmbedSourceId` is the per-node decision the production
+ * `duplicatePage` callback now calls directly, so these tests guard the real
+ * path rather than a parallel copy. `remapPageEmbedSourceIds` is the JSON
+ * walker that delegates to the same helper; its tests exercise the shared
+ * decision transitively across nested ProseMirror containers.
+ */
+describe('remapPageEmbedSourceId (shared per-node decision used by duplicatePage)', () => {
+  it('returns the new copy id when the source IS in the copied set', () => {
+    const idMap = new Map([['old-src', 'new-copy']]);
+
+    const out = remapPageEmbedSourceId('old-src', (id) => idMap.get(id));
+
+    expect(out).toBe('new-copy');
+  });
+
+  it('returns the original id when the source is NOT in the copied set', () => {
+    const idMap = new Map([['old-src', 'new-copy']]);
+
+    const out = remapPageEmbedSourceId('external', (id) => idMap.get(id));
+
+    expect(out).toBe('external');
+  });
+
+  it('returns the original id when resolveNewId yields undefined', () => {
+    const out = remapPageEmbedSourceId('some-id', () => undefined);
+
+    expect(out).toBe('some-id');
+  });
+
+  it('leaves a null source unchanged without consulting the resolver', () => {
+    const resolve = jest.fn(() => 'should-not-be-used');
+
+    const out = remapPageEmbedSourceId(null, resolve);
+
+    expect(out).toBeNull();
+    expect(resolve).not.toHaveBeenCalled();
+  });
+
+  it('leaves an undefined source unchanged without consulting the resolver', () => {
+    const resolve = jest.fn(() => 'should-not-be-used');
+
+    const out = remapPageEmbedSourceId(undefined, resolve);
+
+    expect(out).toBeUndefined();
+    expect(resolve).not.toHaveBeenCalled();
+  });
+});
+
+describe('remapPageEmbedSourceIds (duplicatePage pageEmbed remap)', () => {
+  const docWithEmbeds = (ids: string[]) => ({
+    type: 'doc',
+    content: ids.map((id) => ({
+      type: 'pageEmbed',
+      attrs: { sourcePageId: id },
+    })),
+  });
+
+  it('remaps a source that IS within the copied set to its new copy id', () => {
+    const doc = docWithEmbeds(['old-src']);
+    const idMap = new Map([['old-src', 'new-copy']]);
+
+    const out = remapPageEmbedSourceIds(doc, idMap);
+
+    expect(out.content[0].attrs.sourcePageId).toBe('new-copy');
+  });
+
+  it('keeps the original id for a source NOT in the copied set', () => {
+    const doc = docWithEmbeds(['external']);
+    const idMap = new Map([['old-src', 'new-copy']]); // does not contain "external"
+
+    const out = remapPageEmbedSourceIds(doc, idMap);
+
+    expect(out.content[0].attrs.sourcePageId).toBe('external');
+  });
+
+  it('handles a mixed doc: in-set remapped, out-of-set preserved', () => {
+    const doc = docWithEmbeds(['in-set', 'external']);
+    const idMap = new Map([['in-set', 'in-set-copy']]);
+
+    const out = remapPageEmbedSourceIds(doc, idMap);
+
+    expect(out.content.map((n: any) => n.attrs.sourcePageId)).toEqual([
+      'in-set-copy',
+      'external',
+    ]);
+  });
+
+  it('remaps pageEmbeds nested inside columns', () => {
+    const doc = {
+      type: 'doc',
+      content: [
+        {
+          type: 'columnList',
+          content: [
+            {
+              type: 'column',
+              content: [
+                { type: 'pageEmbed', attrs: { sourcePageId: 'nested-in' } },
+              ],
+            },
+            {
+              type: 'column',
+              content: [
+                { type: 'pageEmbed', attrs: { sourcePageId: 'nested-out' } },
+              ],
+            },
+          ],
+        },
+      ],
+    };
+    const idMap = new Map([['nested-in', 'nested-in-copy']]);
+
+    const out = remapPageEmbedSourceIds(doc, idMap) as any;
+
+    const col0 = out.content[0].content[0].content[0];
+    const col1 = out.content[0].content[1].content[0];
+    expect(col0.attrs.sourcePageId).toBe('nested-in-copy');
+    expect(col1.attrs.sourcePageId).toBe('nested-out');
+  });
+
+  it('remaps pageEmbeds nested inside a callout', () => {
+    const doc = {
+      type: 'doc',
+      content: [
+        {
+          type: 'callout',
+          content: [
+            { type: 'pageEmbed', attrs: { sourcePageId: 'in-callout' } },
+          ],
+        },
+      ],
+    };
+    const idMap = new Map([['in-callout', 'in-callout-copy']]);
+
+    const out = remapPageEmbedSourceIds(doc, idMap) as any;
+
+    expect(out.content[0].content[0].attrs.sourcePageId).toBe(
+      'in-callout-copy',
+    );
+  });
+
+  it('does not descend into a transclusionSource (schema-isolated)', () => {
+    const doc = {
+      type: 'doc',
+      content: [
+        {
+          type: 'transclusionSource',
+          attrs: { id: 'src' },
+          content: [
+            { type: 'pageEmbed', attrs: { sourcePageId: 'hidden' } },
+          ],
+        },
+      ],
+    };
+    const idMap = new Map([['hidden', 'should-not-apply']]);
+
+    const out = remapPageEmbedSourceIds(doc, idMap) as any;
+
+    // The embed inside a source must be left untouched.
+    expect(out.content[0].content[0].attrs.sourcePageId).toBe('hidden');
+  });
+
+  it('leaves embeds missing a sourcePageId untouched', () => {
+    const doc = {
+      type: 'doc',
+      content: [
+        { type: 'pageEmbed', attrs: {} },
+        { type: 'pageEmbed', attrs: { sourcePageId: '' } },
+      ],
+    };
+    const idMap = new Map([['', 'x']]);
+
+    const out = remapPageEmbedSourceIds(doc, idMap) as any;
+
+    expect(out.content[0].attrs.sourcePageId).toBeUndefined();
+    expect(out.content[1].attrs.sourcePageId).toBe('');
+  });
+
+  it('returns the doc unchanged when idMap is empty', () => {
+    const doc = docWithEmbeds(['a', 'b']);
+
+    const out = remapPageEmbedSourceIds(doc, new Map());
+
+    expect(out.content.map((n: any) => n.attrs.sourcePageId)).toEqual([
+      'a',
+      'b',
+    ]);
+  });
+});

apps/server/src/core/page/transclusion/spec/page-template-references-sync.spec.ts

@@ -0,0 +1,310 @@
+import { TransclusionService } from '../transclusion.service';
+
+/**
+ * Covers two untested, high-risk write paths around `page_template_references`:
+ *
+ *  1. `syncPageTemplateReferences` — the `toDelete` branch: stale references are
+ *     removed when the host page no longer embeds a source, while genuinely new
+ *     embeds are inserted. We assert `deleteByReferenceAndSources` /  `insertMany`
+ *     receive the correct rows and the returned `{ inserted, deleted }` counts.
+ *
+ *  2. `insertTemplateReferencesForPages` — the multi-workspace grouping/filtering
+ *     branch: candidate source ids are grouped per workspace, each workspace is
+ *     validated independently, and cross-workspace sources are dropped.
+ *
+ * Setup/mocking mirrors the existing transclusion specs (page-template-access /
+ * page-template-lookup): `new TransclusionService(...)` is built with the same
+ * 11 positional mock args; only the deps each test touches are real stubs.
+ */
+
+/**
+ * Chainable kysely `db` stub used by `filterInWorkspaceSourceIds`. Every
+ * `selectFrom(...).select(...).where(...)` returns the same builder; `.execute()`
+ * resolves whatever rows the per-call resolver returns. The resolver receives
+ * the captured `where('id','in', <ids>)` and `where('workspaceId','=', ws)`
+ * arguments so a test can decide, per workspace, which ids "exist".
+ */
+function makeWorkspaceScopedDb(
+  resolve: (ids: string[], workspaceId: string) => string[],
+) {
+  const state = { ids: [] as string[], workspaceId: '' };
+  const builder: any = {};
+  builder.selectFrom = jest.fn(() => builder);
+  builder.select = jest.fn(() => builder);
+  builder.where = jest.fn((col: string, _op: string, val: any) => {
+    if (col === 'id') state.ids = val as string[];
+    if (col === 'workspaceId') state.workspaceId = val as string;
+    return builder;
+  });
+  builder.execute = jest.fn(async () =>
+    resolve(state.ids, state.workspaceId).map((id) => ({ id })),
+  );
+  return builder;
+}
+
+function buildService(opts: {
+  db: any;
+  pageTemplateReferencesRepo: any;
+}) {
+  return new TransclusionService(
+    opts.db,
+    {} as any, // pageTransclusionsRepo
+    {} as any, // pageTransclusionReferencesRepo
+    opts.pageTemplateReferencesRepo,
+    {} as any, // pageRepo
+    {} as any, // pagePermissionRepo
+    {} as any, // spaceMemberRepo
+    {} as any, // attachmentRepo
+    {} as any, // storageService
+    {} as any, // pageAccessService
+    {} as any, // workspaceRepo
+  );
+}
+
+const pageEmbedDoc = (sourceIds: string[]) => ({
+  type: 'doc',
+  content: sourceIds.map((id) => ({
+    type: 'pageEmbed',
+    attrs: { sourcePageId: id },
+  })),
+});
+
+describe('TransclusionService.syncPageTemplateReferences — toDelete branch', () => {
+  it('deletes stale references and inserts new ones with correct args/counts', async () => {
+    // Every candidate id is treated as in-workspace by the existence query.
+    const db = makeWorkspaceScopedDb((ids) => ids);
+
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const deleteByReferenceAndSources = jest.fn().mockResolvedValue(undefined);
+    const pageTemplateReferencesRepo = {
+      // existing refs: "keep" stays embedded, "stale-a"/"stale-b" no longer are
+      findByReferencePageId: jest.fn().mockResolvedValue([
+        { sourcePageId: 'keep' },
+        { sourcePageId: 'stale-a' },
+        { sourcePageId: 'stale-b' },
+      ]),
+      insertMany,
+      deleteByReferenceAndSources,
+    };
+
+    const service = buildService({ db, pageTemplateReferencesRepo });
+
+    // host now embeds: keep (unchanged) + fresh (new). stale-a/stale-b gone.
+    const result = await service.syncPageTemplateReferences(
+      'host',
+      'w1',
+      pageEmbedDoc(['keep', 'fresh']),
+    );
+
+    expect(result).toEqual({ inserted: 1, deleted: 2 });
+
+    // only the genuinely new embed is inserted (keep already existed)
+    expect(insertMany).toHaveBeenCalledTimes(1);
+    expect(insertMany.mock.calls[0][0]).toEqual([
+      { workspaceId: 'w1', referencePageId: 'host', sourcePageId: 'fresh' },
+    ]);
+
+    // stale references removed, scoped to host + workspace
+    expect(deleteByReferenceAndSources).toHaveBeenCalledTimes(1);
+    const [refPageId, workspaceId, staleSources] =
+      deleteByReferenceAndSources.mock.calls[0];
+    expect(refPageId).toBe('host');
+    expect(workspaceId).toBe('w1');
+    expect([...staleSources].sort()).toEqual(['stale-a', 'stale-b']);
+  });
+
+  it('deletes ALL existing references when the host embeds nothing anymore', async () => {
+    const db = makeWorkspaceScopedDb((ids) => ids);
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const deleteByReferenceAndSources = jest.fn().mockResolvedValue(undefined);
+    const pageTemplateReferencesRepo = {
+      findByReferencePageId: jest
+        .fn()
+        .mockResolvedValue([{ sourcePageId: 'a' }, { sourcePageId: 'b' }]),
+      insertMany,
+      deleteByReferenceAndSources,
+    };
+
+    const service = buildService({ db, pageTemplateReferencesRepo });
+
+    const result = await service.syncPageTemplateReferences(
+      'host',
+      'w1',
+      pageEmbedDoc([]), // no embeds left
+    );
+
+    expect(result).toEqual({ inserted: 0, deleted: 2 });
+    expect(insertMany).not.toHaveBeenCalled();
+    const [, , staleSources] = deleteByReferenceAndSources.mock.calls[0];
+    expect([...staleSources].sort()).toEqual(['a', 'b']);
+  });
+
+  it('treats a cross-workspace embed as stale: it never survives to be kept', async () => {
+    // existence query drops "cross-ws"; so an existing ref to it must be deleted
+    const db = makeWorkspaceScopedDb((ids) => ids.filter((id) => id !== 'cross-ws'));
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const deleteByReferenceAndSources = jest.fn().mockResolvedValue(undefined);
+    const pageTemplateReferencesRepo = {
+      findByReferencePageId: jest
+        .fn()
+        .mockResolvedValue([{ sourcePageId: 'cross-ws' }]),
+      insertMany,
+      deleteByReferenceAndSources,
+    };
+
+    const service = buildService({ db, pageTemplateReferencesRepo });
+
+    // host still "embeds" cross-ws in its doc, but it is not in-workspace
+    const result = await service.syncPageTemplateReferences(
+      'host',
+      'w1',
+      pageEmbedDoc(['cross-ws']),
+    );
+
+    expect(result).toEqual({ inserted: 0, deleted: 1 });
+    expect(insertMany).not.toHaveBeenCalled();
+    const [, , staleSources] = deleteByReferenceAndSources.mock.calls[0];
+    expect([...staleSources]).toEqual(['cross-ws']);
+  });
+
+  it('no-ops both repos when desired and existing already match', async () => {
+    const db = makeWorkspaceScopedDb((ids) => ids);
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const deleteByReferenceAndSources = jest.fn().mockResolvedValue(undefined);
+    const pageTemplateReferencesRepo = {
+      findByReferencePageId: jest
+        .fn()
+        .mockResolvedValue([{ sourcePageId: 'same' }]),
+      insertMany,
+      deleteByReferenceAndSources,
+    };
+
+    const service = buildService({ db, pageTemplateReferencesRepo });
+
+    const result = await service.syncPageTemplateReferences(
+      'host',
+      'w1',
+      pageEmbedDoc(['same']),
+    );
+
+    expect(result).toEqual({ inserted: 0, deleted: 0 });
+    expect(insertMany).not.toHaveBeenCalled();
+    expect(deleteByReferenceAndSources).not.toHaveBeenCalled();
+  });
+});
+
+describe('TransclusionService.insertTemplateReferencesForPages — multi-workspace grouping', () => {
+  it('groups candidates per workspace and validates each workspace independently', async () => {
+    // Each workspace "owns" only its own source ids. The existence query is
+    // workspace-scoped, so an id from another workspace is dropped.
+    const owned: Record<string, string[]> = {
+      w1: ['s1'],
+      w2: ['s2'],
+    };
+    const executeArgs: Array<{ ids: string[]; workspaceId: string }> = [];
+    const db = makeWorkspaceScopedDb((ids, workspaceId) => {
+      executeArgs.push({ ids: [...ids], workspaceId });
+      const ownedSet = new Set(owned[workspaceId] ?? []);
+      return ids.filter((id) => ownedSet.has(id));
+    });
+
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const pageTemplateReferencesRepo = { insertMany };
+
+    const service = buildService({ db, pageTemplateReferencesRepo });
+
+    // page-a in w1 embeds s1 (valid) + s2 (belongs to w2 -> dropped)
+    // page-b in w2 embeds s2 (valid)
+    const result = await service.insertTemplateReferencesForPages([
+      { id: 'page-a', workspaceId: 'w1', content: pageEmbedDoc(['s1', 's2']) },
+      { id: 'page-b', workspaceId: 'w2', content: pageEmbedDoc(['s2']) },
+    ]);
+
+    expect(result).toEqual({ inserted: 2 });
+
+    expect(insertMany).toHaveBeenCalledTimes(1);
+    const rows = insertMany.mock.calls[0][0];
+    expect(rows).toEqual([
+      { workspaceId: 'w1', referencePageId: 'page-a', sourcePageId: 's1' },
+      { workspaceId: 'w2', referencePageId: 'page-b', sourcePageId: 's2' },
+    ]);
+
+    // one existence query per workspace, each scoped to that workspace's candidates
+    expect(executeArgs).toHaveLength(2);
+    const w1Call = executeArgs.find((c) => c.workspaceId === 'w1');
+    const w2Call = executeArgs.find((c) => c.workspaceId === 'w2');
+    expect(w1Call?.ids.sort()).toEqual(['s1', 's2']);
+    expect(w2Call?.ids).toEqual(['s2']);
+  });
+
+  it('drops every cross-workspace source and inserts nothing when none are in-workspace', async () => {
+    // No id is owned by its page's workspace -> all filtered out.
+    const db = makeWorkspaceScopedDb(() => []);
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const service = buildService({
+      db,
+      pageTemplateReferencesRepo: { insertMany },
+    });
+
+    const result = await service.insertTemplateReferencesForPages([
+      { id: 'page-a', workspaceId: 'w1', content: pageEmbedDoc(['x']) },
+      { id: 'page-b', workspaceId: 'w2', content: pageEmbedDoc(['y']) },
+    ]);
+
+    expect(result).toEqual({ inserted: 0 });
+    expect(insertMany).not.toHaveBeenCalled();
+  });
+
+  it('dedupes a sourceId shared by two pages in the same workspace into one validation', async () => {
+    const executeArgs: Array<{ ids: string[]; workspaceId: string }> = [];
+    const db = makeWorkspaceScopedDb((ids, workspaceId) => {
+      executeArgs.push({ ids: [...ids], workspaceId });
+      return ids; // all in-workspace
+    });
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const service = buildService({
+      db,
+      pageTemplateReferencesRepo: { insertMany },
+    });
+
+    // both pages embed the same source "shared" in w1
+    const result = await service.insertTemplateReferencesForPages([
+      { id: 'page-a', workspaceId: 'w1', content: pageEmbedDoc(['shared']) },
+      { id: 'page-b', workspaceId: 'w1', content: pageEmbedDoc(['shared']) },
+    ]);
+
+    // a row per (page, source) pair, but only one existence query for w1
+    expect(result).toEqual({ inserted: 2 });
+    expect(executeArgs).toHaveLength(1);
+    expect(executeArgs[0]).toEqual({ ids: ['shared'], workspaceId: 'w1' });
+
+    const rows = insertMany.mock.calls[0][0];
+    expect(rows).toEqual([
+      { workspaceId: 'w1', referencePageId: 'page-a', sourcePageId: 'shared' },
+      { workspaceId: 'w1', referencePageId: 'page-b', sourcePageId: 'shared' },
+    ]);
+  });
+
+  it('returns inserted:0 without querying when no page has embeds', async () => {
+    const execute = jest.fn();
+    const db = makeWorkspaceScopedDb(() => {
+      execute();
+      return [];
+    });
+    const insertMany = jest.fn().mockResolvedValue(undefined);
+    const service = buildService({
+      db,
+      pageTemplateReferencesRepo: { insertMany },
+    });
+
+    const result = await service.insertTemplateReferencesForPages([
+      { id: 'page-a', workspaceId: 'w1', content: pageEmbedDoc([]) },
+    ]);
+
+    expect(result).toEqual({ inserted: 0 });
+    expect(insertMany).not.toHaveBeenCalled();
+    // filterInWorkspaceSourceIds short-circuits on empty candidates, so the
+    // existence query never runs.
+    expect(execute).not.toHaveBeenCalled();
+  });
+});

apps/server/src/core/page/transclusion/utils/transclusion-prosemirror.util.ts

@@ -99,6 +99,64 @@ export function collectReferencesFromPmJson(
   return out;
 }
 
+/**
+ * Decide the sourcePageId a duplicated pageEmbed should point to: the copy's new
+ * id when the embedded source is part of the copied set, otherwise the original
+ * (a live embed of the original page). Pure — shared by PageService.duplicatePage
+ * (the real path) and the JSON walker below, so both stay in lockstep.
+ */
+export function remapPageEmbedSourceId(
+  sourcePageId: string | null | undefined,
+  resolveNewId: (id: string) => string | undefined,
+): string | null | undefined {
+  if (sourcePageId) {
+    const mapped = resolveNewId(sourcePageId);
+    if (mapped) return mapped;
+  }
+  return sourcePageId;
+}
+
+/**
+ * Remap the `sourcePageId` of every `pageEmbed` node in a ProseMirror JSON doc
+ * according to `idMap` (old page id -> new page id). Delegates the per-node
+ * decision to the shared `remapPageEmbedSourceId` helper that
+ * `PageService.duplicatePage` also uses, so the production path and this walker
+ * stay in lockstep: when the embedded source page is part of the copied set
+ * (present in `idMap`) the embed is pointed at its new copy; otherwise the
+ * original `sourcePageId` is preserved so it stays a live embed of the original
+ * page. Mutates `doc` in place (and returns it) to match the service's in-place
+ * ProseMirror mutation. Recurses through arbitrary block containers (columns,
+ * callouts, etc.) the same way the collectors do, but does NOT descend into a
+ * `transclusionSource` (schema-isolated).
+ */
+export function remapPageEmbedSourceIds<T>(
+  doc: T,
+  idMap: Map<string, string>,
+): T {
+  const visit = (node: any): void => {
+    if (!node || typeof node !== 'object') return;
+
+    if (node.type === PAGE_EMBED_TYPE) {
+      if (node.attrs) {
+        node.attrs.sourcePageId = remapPageEmbedSourceId(
+          node.attrs.sourcePageId,
+          (id) => idMap.get(id),
+        );
+      }
+      return; // atom node - no children
+    }
+
+    if (node.type === TRANSCLUSION_TYPE) return;
+
+    if (Array.isArray(node.content)) {
+      for (const child of node.content) visit(child);
+    }
+  };
+
+  visit(doc);
+  return doc;
+}
+
 /**
  * Walks a ProseMirror JSON document and returns one snapshot per unique
  * `sourcePageId` found on `pageEmbed` nodes (whole-page live embeds). Order