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).

src/server/chat-handler.ts

@@ -4,6 +4,7 @@ import { createProberAgent, createTeacherAgent, type TeachingAgent } from "../ag
 import { piEngineFor } from "../agent/models.ts";
 import { createJobRunner } from "../agent/subagents/runner.ts";
 import { createExtractionPipeline } from "../agent/subagents/extraction.ts";
+import { embedNewNodes } from "./embeddings.ts";
 import type { ModelRole } from "../../config.ts";
 import { claimSteers, deliverJobResult, setJobRespawn, setTurnRunner } from "./jobs.ts";
 import { recordToPiMessages } from "./pi/record-adapter.ts";
@@ -40,7 +41,12 @@ const jobRunner = createJobRunner({
 /** The app's ONE extraction pipeline instance — both front doors share it:
  *  quiz-triggered session runs (turn-service) and notes imports
  *  (/api/graph/import) hit the same single-flight registry. */
-export const extraction = createExtractionPipeline(stores);
+export const extraction = createExtractionPipeline({
+	...stores,
+	// The REAL embedder (local transformer model): post-merge embedding
+	// batch — semantic clusters are computed and persisted here.
+	embedder: embedNewNodes,
+});
 
 // Wake-pipeline wiring (contracts.ts "THE ASYNC PROTOCOL" §4): the pipeline
 // fires its server-side teacher turns through THIS composition root, and