spec(UseCaseCatalog): fix open-question numbering + add marker legend

- Renumber stray open-question 23->31 (mis-numbered during the WP-0003 batch;
  questions now run 1-32 sequentially) and add Q32 (federation composition,
  from synthesis v3 S2.5).
- Add a dive-source marker legend at the top of Traceability (18 markers were
  used across the matrix with no single key).

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

spec/UseCaseCatalog.md

@@ -1383,6 +1383,19 @@ identities are part of the legacy identity surface the adapter must parse and tr
 
 ## Traceability
 
+Dive-source markers (placed in the nearest existing column — usually **wikiengines** or
+**federation** — since the 260613/260614 deep dives postdate the original source columns; a
+marker's true lineage is its per-source mapping subsection below):
+
+| Marker | Dive | Marker | Dive | Marker | Dive |
+|--------|------|--------|------|--------|------|
+| † | xanadu | ‡ | zigzag | § | roam |
+| ¶ | obsidian | ◊ | notion | ‖ | joplin |
+| ※ | logseq | ⌘ | localfirst (Anytype/AFFiNE/AppFlowy) | ⊕ | trilium |
+| ⚓ | wiki.js | ⊞ | federated-wiki | ⬡ | wikibase |
+| ⎇ | forge-wikis (Gitea/GitLab/GitHub) | ⊡ | tiddlywiki | ⊟ | ikiwiki |
+| ◧ | quip | ⊙ | mojomojo | ⊚ | oddmuse (UseModWiki reinforces, no marker) |
+
 | UC | c2 research | yawex research | federation research | wikiengines research | INTENT |
 |----|-------------|----------------|---------------------|----------------------|--------|
 | UC-01 | ✓ | | | | ✓ |
@@ -2194,5 +2207,9 @@ identity surface (UC-25) the adapter must parse/translate.
 30. How does shard-wiki represent a shard with **partial/truncated history** (UC-82, Oddmuse
     `keep/` expiry) in the journal and provenance UI — an explicit "history begins at"
     marker / completeness metadata? (Oddmuse dive §7.)
-23. How does shard-wiki **honor/surface a shard's path-based access rules** (UC-06) in a
-    projection without re-implementing its ACL engine? (Wiki.js dive §9.)
+31. How does shard-wiki **honor/surface a shard's path-based access rules** (UC-06) in a
+    projection without re-implementing its ACL engine? (Wiki.js dive §9.)
+32. **Federation composition** — can one information space run several federation models
+    (fork+journal / VCS-replication+ping / query-time graph join / engine-mirror, synthesis
+    v3 §2.5) concurrently over different shards, and how does the union reconcile a chorus
+    (UC-72) with a canonical-asserting shard? (Synthesis v3 §6.)