refactor(transclusion): unify the ProseMirror collectors into collectNodes (#94)

The three collect*FromPmJson collectors shared the same recursion (and the #55
depth cap) but were copy-pasted, so a future edit could diverge them. Extract a
generic collectNodes(doc, {type, map, key, lastWins, skipChildrenOfType}) and
reimplement all three on it, byte-output-identical (transclusions last-wins;
references/embeds first-wins + transclusionSource skip). Documents (not removes)
the write-only page_template_references graph and the near-duplicate client
lookup-context as tracked follow-ups, per the issue's guidance.

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

apps/client/src/features/editor/components/page-embed/page-embed-lookup-context.tsx

@@ -25,6 +25,15 @@ const PageEmbedLookupContext = createContext<ContextValue | null>(null);
  * transclusion lookup context but keys purely on `sourcePageId`. On public
  * shares there is no lookup in MVP, so the context simply isn't mounted (the
  * node view renders a placeholder when the context is absent).
+ *
+ * NOTE (intentional near-duplicate of `transclusion-lookup-context.tsx`): this
+ * provider duplicates that file's batching / de-dup / cache machinery; only the
+ * lookup key (sourcePageId here vs sourcePageId+transclusionId there) and the
+ * API call differ. Unifying them now would mean a generic, parameterised lookup
+ * provider — a larger client refactor that isn't worth it for just two
+ * consumers. Per Gitea #94, extract a shared generic provider when a THIRD
+ * lookup consumer appears; until then keep the two in sync by hand. (Tracked,
+ * deliberately deferred — not forgotten.)
  */
 export function PageEmbedLookupProvider({
   children,