fix(footnotes): strip bare definitions on rebuild; MCP full-doc + zip-import canonicalize tests (#228)

Review #6 (approve-with-comments) follow-ups:
1. canonicalize step 7 now strips bare footnoteDefinitions at ANY depth
   (stripFootnoteDefinitionsDeep), not just footnotesList, in BOTH copies. A
   definition hand-authored outside a list (e.g. nested in a callout via a
   raw-JSON write path) was left in place while a copy was also added to the
   rebuilt list -> duplicate, idempotent, self-perpetuating. Runs only in the
   rebuild path (after the lists are stripped); the fast-path / placement-keep
   branch is untouched. Added a shared-corpus case (bare def nested in a callout)
   to pin it in both mirrors.
2. markdown-clipboard: removed the dead top-level footnoteReference check in
   canonicalizePastedFootnotes (an inline atom is never a top-level slice child;
   only the descendants scan can find it).

Test coverage:
4. New MCP binding tests (full-doc-write-canonicalize.test.mjs): update_page_json
   and copy_page_content canonicalize the persisted full doc, asserted via a new
   `replacePage` seam (symmetric to the existing `mutatePage` seam) so no live
   collab socket is needed. Routed both writers through the seam.
5. New server spec (file-import-task.service.footnote-canonicalize.spec.ts): the
   zip-import path (processGenericImport) canonicalizes footnotes — real
   markdown->HTML->JSON via a real ImportService over a temp-dir .md file, DB trx
   stubbed to capture the persisted page content. FileImportTaskService had no
   spec before.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/editor/extensions/markdown-clipboard.ts

@@ -179,7 +179,9 @@ export function canonicalizePastedFootnotes(slice: Slice, schema: Schema): Slice
   let hasReference = false;
   slice.content.forEach((node) => {
     if (node.type.name === FOOTNOTES_LIST_NAME) hasFootnotesList = true;
-    if (node.type.name === FOOTNOTE_REFERENCE_NAME) hasReference = true;
+    // footnoteReference is an inline atom, never a top-level slice child here
+    // (this function early-returns for open slices, so children are whole
+    // blocks), so it is only reachable by descending.
     node.descendants((child) => {
       if (child.type.name === FOOTNOTE_REFERENCE_NAME) hasReference = true;
     });