Semantic topic clusters: local embeddings replace Louvain

Louvain only ever found the reasoning chains (the graph is disconnected
by nature — every connected component became its own cluster) and
edge-orphans could never join anything. Clusters are now RELATED TOPICS:
propositions grouped by meaning.

- Embedder (src/server/embeddings.ts): in-process transformers.js v4
  running EmbeddingGemma-300M q8 (MTEB 68.4, ~300MB one-time download),
  MRL-truncated to 512d, "title: none | text:" prefix on 100% of inputs,
  L2-normalized so cosine = dot. Model/dim mismatch voids the ledger
  (full re-embed). Injected at the composition root — the agent layer
  never imports the server layer (architecture law).
- Ledger (data/graph/embeddings.json): model + dim + vectors + the
  persisted cluster partition. Reclustering runs at the END of every
  embedding batch (post-merge hook, both front doors) — never per read
  (the agglomerative pass is ~1.5s at 338 nodes).
- Clustering (src/core/semantic-clusters.ts): hand-rolled average-linkage
  agglomerative over cosine (npm HDBSCAN is an abandoned alpha). Stragglers
  stay singletons by default; attachStragglers is the all-in toggle.
  Threshold 0.60 swept on the real graph: 26 topics, 74/90 edge-orphans
  land in real topics, 10 stragglers.
- Consumers (snapshot, settle, namer, replay) read the persisted
  partition — one cluster concept everywhere; core-survival label
  matching unchanged (20/26 topics kept labels).
- Louvain retired: detectCommunities + graphology* deps deleted
  (graphology-louvain had leaked into the client bundle since the
  clusters wave); scripts/bench-graph.ts (dead FA2 bench) removed.
- scripts/embed-graph.ts: backfill/cold-start (338 nodes in 150s
  including the one-time model download).

components/graph-view.tsx

@@ -14,10 +14,10 @@
  * per-camera-event full-canvas repaints (GPU spikes + coil whine) are
  * gone (at most one draw per frame, and NO frame at all while idle).
  *
- * Layout (graphology + forceAtlas2) runs-then-freezes exactly as
- * before: ONE synchronous 100-iteration pass inside a useEffect writes
- * world-space x/y, then the positions are handed to the renderer as
- * data. No layout animation ever starts; the only animation is the
+ * Layout fallback (ledger absent): ONE synchronous incrementalLayout
+ * pass (d3-force, core/layout.ts) inside a useEffect writes world-space
+ * x/y, then the positions are handed to the renderer as data. No layout
+ * animation ever starts; the only animation is the
  * short hover fade, strictly bounded in time — idle CPU is zero (the
  * loop schedules no rAF until something actually changes).
  *