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).

scripts/replay-layout.ts

@@ -13,7 +13,6 @@
  * playground mode baked into the rest state); omit it for the free layout.
  */
 import { graphStore } from "../src/store/graph.ts";
-import { detectCommunities } from "../src/core/clusters.ts";
 // --depth=vertical|radial: bake the playground depth mode into the ledger.
 const depthArg = process.argv.find((a) => a.startsWith("--depth="));
 const depthMode: "off" | "vertical" | "radial" =
@@ -33,6 +32,12 @@ function main() {
 		nodes.push(line);
 	}
 	const edges = graphStore.readEdges();
+	// The semantic-topic source: the embedding ledger (run
+	// scripts/embed-graph.ts first if missing/stale).
+	const embeddings = graphStore.readEmbeddings();
+	if (Object.keys(embeddings.vectors).length === 0) {
+		console.warn("[replay] embedding ledger is empty — clusters will be absent (run scripts/embed-graph.ts)");
+	}
 
 	// Batches: each imported file, in real import order (sources.json
 	// importedAt). A node belongs to the file whose mapping first claims it
@@ -79,8 +84,12 @@ function main() {
 		const before = new Map(positions);
 		const batchNodes = nodes.filter((n) => delivered.has(n.id));
 		positions = incrementalLayout(batchNodes, activeEdges, positions, {
-			// The same partition the real settle would see for this batch.
-			clusters: detectCommunities(batchNodes, activeEdges).communities,
+			// The same partition the real settle would see for this batch:
+			// the persisted semantic topics, filtered to delivered nodes
+			// (never recomputed — the agglomerative pass is ~1.5s at scale).
+			clusters: embeddings.clusters
+				.map((c) => ({ members: c.members.filter((id) => delivered.has(id)) }))
+				.filter((c) => c.members.length > 0),
 			...(depthMode === "off"
 				? {}
 				: {