perf+fix(footnotes): minimal-diff sync (no concurrent-edit loss); cache numbering

Release-cycle review found two hardening gaps:
- The sync plugin deleted+rebuilt the WHOLE footnotesList on any reorder/orphan,
  replacing every definition's Yjs subtree -> a collaborator typing in a
  definition could lose in-flight characters on merge. Rework to targeted,
  minimal mutations: attr-only setNodeMarkup for collision re-ids, delete only
  genuine orphans, insert only genuinely-missing definitions (at the list end,
  not shifting existing subtrees), and consolidate multiple lists only in the
  abnormal paste/merge case. An unchanged (correct id, referenced) definition is
  left completely untouched. Numbering is decoration-only, so physical list order
  may drift after a reorder (accepted) while displayed numbers stay correct.
  Invariants preserved (reviewed + tested): one SYNC_META transaction, null when
  canonical (terminates), deterministic deriveFootnoteId, remote-skip -> no
  re-introduced freeze or divergence.
- computeFootnoteNumbers ran per-NodeView-render (O(n^2)/keystroke in big docs).
  The numbering plugin now caches the number map in its state (computed once per
  docChanged); NodeViews read it O(1) via getFootnoteNumber.

Tests: no-rebuild-on-reorder asserts unchanged definition node subtrees are
identity-preserved; isRemoteTransaction skip; enableSync:false read-only; cache
correctness. Browser re-smoke: insert (no freeze), number, persist across reload,
cascade delete all pass.

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

apps/client/src/features/editor/components/footnote/footnote-definition-view.tsx

@@ -1,6 +1,6 @@
 import { NodeViewContent, NodeViewProps, NodeViewWrapper } from "@tiptap/react";
 import { useTranslation } from "react-i18next";
-import { computeFootnoteNumbers } from "@docmost/editor-ext";
+import { getFootnoteNumber } from "@docmost/editor-ext";
 import classes from "./footnote.module.css";
 
 /**
@@ -13,8 +13,9 @@ export default function FootnoteDefinitionView(props: NodeViewProps) {
   const { t } = useTranslation();
   const id = node.attrs.id as string;
 
-  const numbers = computeFootnoteNumbers(editor.state.doc);
-  const number = numbers.get(id) ?? "?";
+  // Read the cached number from the numbering plugin (computed once per doc
+  // change) rather than recomputing the whole map on every render.
+  const number = getFootnoteNumber(editor.state, id) ?? "?";
 
   const handleBack = (e: React.MouseEvent) => {
     e.preventDefault();

apps/client/src/features/editor/components/footnote/footnote-reference-view.tsx

@@ -11,7 +11,7 @@ import {
 } from "@floating-ui/dom";
 import {
   FOOTNOTE_DEFINITION_NAME,
-  computeFootnoteNumbers,
+  getFootnoteNumber,
 } from "@docmost/editor-ext";
 import { ActionIcon } from "@mantine/core";
 import { IconArrowDown } from "@tabler/icons-react";
@@ -45,9 +45,10 @@ export default function FootnoteReferenceView(props: NodeViewProps) {
   const popoverRef = useRef<HTMLDivElement | null>(null);
   const [open, setOpen] = useState(false);
 
-  // Number is derived (not stored) — recompute from the current doc.
-  const numbers = computeFootnoteNumbers(editor.state.doc);
-  const number = numbers.get(id) ?? "?";
+  // Number is derived (not stored). Read it from the numbering plugin's cached
+  // map (computed once per doc change) instead of walking the whole document on
+  // every render — recomputing per NodeView per render was O(n^2) per keystroke.
+  const number = getFootnoteNumber(editor.state, id) ?? "?";
   const defText = open ? getDefinitionText(editor, id) : "";
 
   const position = useCallback(() => {