fix(footnotes): survive duplicate-id definitions without collab divergence

Release-cycle red-team found two same-id footnoteDefinition nodes (trivially
produced by markdown import [^d]: first / [^d]: second, or paste/duplicate)
caused silent data loss: scan() used a last-wins Map and the sync rebuild
(addToHistory:false, propagated via Yjs, un-undoable) dropped all but the last.

Fix resolves collisions so BOTH survive, with a DETERMINISTIC id scheme so
collaborators converge:
- deriveFootnoteId(originalId, occurrence, taken): the k-th (k>=2) occurrence of
  id X becomes X__k, bumped with a deterministic alpha suffix only against the
  doc's own id set — a pure function of document state. No Math.random/Date.now
  on the sync or import paths (random uuid stays only in setFootnote, where a
  single user originates a brand-new id).
- footnote-sync.resolveCollisions walks refs+defs in document order, re-ids
  duplicate references via setNodeMarkup and pairs them 1:1 with definitions;
  single SYNC_META-tagged transaction, returns null when canonical (terminates).
- Markdown import (footnote.marked) + MCP mirror (collaboration.ts) dedup with
  the same deterministic scheme + marker rewrite; packages/mcp/build regenerated.
- Paste plugin remaps colliding pasted ids against the current doc.

Tests: two independent editors resolving the same duplicate-id doc produce
IDENTICAL ids (the cross-client determinism guard that the random version would
fail); both definitions survive the first edit; import dedup is deterministic.

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

packages/mcp/build/lib/collaboration.js

@@ -271,6 +271,44 @@ const FOOTNOTE_REF_RE = /\[\^([^\]\s]+)\]/;
 function escapeFootnoteAttr(value) {
     return String(value).replace(/&/g, "&").replace(/"/g, """);
 }
+function escapeFootnoteRegExp(value) {
+    return value.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+/**
+ * Derive a DETERMINISTIC unique footnote id for the k-th (k >= 2) occurrence of
+ * an original id `X` during definition dedup.
+ *
+ * EXACT MIRROR of editor-ext `deriveFootnoteId`
+ * (packages/editor-ext/src/lib/footnote/footnote-util.ts). These two copies MUST
+ * STAY IN SYNC: the same markdown imported through the editor and through this
+ * MCP path has to produce identical ids, and the sync plugin (which re-ids on
+ * every collaborating client) relies on the same scheme to converge. NEVER use
+ * Math.random()/Date.now()/uuid here — a random id would diverge across clients.
+ *
+ * Scheme: base candidate `${originalId}__${occurrence}` (e.g. `X__2`), bumped
+ * with a stable alphabetic suffix (`X__2b`, `X__2c`, ...) until it is not in
+ * `taken` (the set of ids already present / already minted — pure doc state).
+ */
+function deriveFootnoteId(originalId, occurrence, taken) {
+    let candidate = `${originalId}__${occurrence}`;
+    let n = 0;
+    while (taken.has(candidate)) {
+        n += 1;
+        candidate = `${originalId}__${occurrence}${footnoteSuffix(n)}`;
+    }
+    return candidate;
+}
+/** Map 1 -> "b", 2 -> "c", ... (mirror of editor-ext `suffix`). */
+function footnoteSuffix(n) {
+    let out = "";
+    let x = n;
+    while (x > 0) {
+        const rem = (x - 1) % 25;
+        out = String.fromCharCode(98 + rem) + out; // 98 = 'b'
+        x = Math.floor((x - 1) / 25);
+    }
+    return out;
+}
 const footnoteRefMarkedExtension = {
     name: "footnoteRef",
     level: "inline",
@@ -319,11 +357,43 @@ function extractFootnotes(markdown) {
     }
     if (defs.length === 0)
         return { body: markdown, section: "" };
+    // De-duplicate colliding definition ids (mirror of editor-ext
+    // extractFootnoteDefinitions). Two definitions sharing an id would otherwise
+    // collapse into one footnote downstream; rename each colliding id to a
+    // DETERMINISTIC derived one (NOT random) and rewrite the corresponding `[^id]`
+    // marker so the (reference, definition) pairing stays 1:1. Determinism lets
+    // the same markdown imported here and via the editor produce identical ids.
+    let dedupedBody = bodyLines.join("\n");
+    const taken = new Set(defs.map((d) => d.id));
+    const seenDefIds = new Map();
+    for (const def of defs) {
+        const originalId = def.id;
+        const count = seenDefIds.get(originalId) ?? 0;
+        seenDefIds.set(originalId, count + 1);
+        if (count === 0)
+            continue; // first definition keeps its id
+        const newId = deriveFootnoteId(originalId, count + 1, taken);
+        taken.add(newId);
+        def.id = newId;
+        // Remaining `[^originalId]` matches: index 0 = keeper's marker (left alone),
+        // index 1 = this duplicate's marker. Rewrite index 1.
+        let occurrence = 0;
+        let rewritten = false;
+        const re = new RegExp(`\\[\\^${escapeFootnoteRegExp(originalId)}\\]`, "g");
+        dedupedBody = dedupedBody.replace(re, (match) => {
+            const idx = occurrence++;
+            if (!rewritten && idx === 1) {
+                rewritten = true;
+                return `[^${newId}]`;
+            }
+            return match;
+        });
+    }
     const inner = defs
         .map((d) => `<div data-footnote-def data-id="${escapeFootnoteAttr(d.id)}"><p>${marked.parseInline(d.text || "")}</p></div>`)
         .join("");
     return {
-        body: bodyLines.join("\n"),
+        body: dedupedBody,
         section: `<section data-footnotes>${inner}</section>`,
     };
 }