docs(p9-fb-23): README + HANDOFF + HOTFIXES + INDEX + per-task spec

Also fixes snapshot drift in code-and-table.canonical.snapshot.json
introduced by task 2 (CanonicalDocument gains last_chunker_version +
last_embedding_version fields).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fixtures/markdown/code-and-table.canonical.snapshot.json

@@ -62,6 +62,8 @@
   "doc_id": "6a9ef317c9c097ff3f6aeb317559bd83",
   "doc_version": 1,
   "lang": "en",
+  "last_chunker_version": null,
+  "last_embedding_version": null,
   "metadata": {
     "aliases": [],
     "created_at": "2023-11-14T22:13:20Z",