Cluster gravity: per-cluster centroid pull force in the engine

The 'virtual node' realized as a centroid force: two allocation-free
O(n) passes per tick (membership resolved once at sim creation;
Float64Array sums). Measured indistinguishable from force-off at 338
nodes (271ms vs 272ms per 400-tick settle).

- createGraphSimulation: SimOptions.clusterPull + linkStrength +
  velocityDecay (physics controls plumbing); degree-scaled link
  stiffness so linkStrength=1 is exactly d3's default
- settleGraphLayout split into layout-settle.ts (it computes the
  Louvain partition for the pull); layout.ts stays browser-barrel-pure
- barrel no longer re-exports clusters.ts: graphology + louvain are
  fully out of the client bundle (verified against built chunks)
- replay passes the per-batch partition; CLUSTER_PULL_STRENGTH=0.15
  tuned by sweep: cluster spread 63->36 world units, centroid
  separation and total span unchanged (no blob merge, no collapse)

src/core/index.ts

@@ -20,7 +20,11 @@ export * from "./ports.ts";
 export * from "./render-settings.ts";
 
 export * from "./question-core.ts";
-export * from "./clusters.ts";
+// clusters.ts (Louvain) is deliberately NOT re-exported: nothing in the
+// shell runs community detection — the snapshot carries the partition
+// from the server. Keeping it out keeps graphology-louvain out of the
+// client bundle (verified 2026-09-13: chunks contained it while this
+// line existed). Same logic as layout-settle.ts below.
 export * from "./layout.ts";
 export * from "./plan-tree.ts";
 export * from "./phases.ts";