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/server/commit-positions.ts

@@ -12,7 +12,7 @@
  */
 import type { Position } from "../core/contracts.ts";
 import { getGraph } from "../core/graph.ts";
-import { settleGraphLayout } from "../core/layout.ts";
+import { settleGraphLayout } from "../core/layout-settle.ts";
 import { stores } from "./stores.ts";
 
 /** Max |x| or |y| accepted — NaN/Infinity clamp here, absurd too. */