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).

next.config.ts

@@ -4,7 +4,13 @@ const nextConfig: NextConfig = {
 	// Framework-free core lives in src/ (plain TS, imported by routes and CLI alike).
 	// The pi engine must stay external: its providers are runtime-loaded Node
 	// code (undici streams, catalog data) that breaks when bundled into routes.
-	serverExternalPackages: ["@earendil-works/pi-ai", "@earendil-works/pi-agent-core"],
+	// transformers.js (embeddings) is runtime-loaded ONNX — same reason as
+	// the pi engine: bundling onnxruntime-node breaks route builds.
+	serverExternalPackages: [
+		"@earendil-works/pi-ai",
+		"@earendil-works/pi-agent-core",
+		"@huggingface/transformers",
+	],
 };
 
 export default nextConfig;