P3-C: Cluster Stability (full implementation)

[github-actions]

Why: Detect and fix unstable clusters (split oversized, merge undersized). The community detection added in P2-F is a subset of this module; here we add the full split/merge machinery.

• P3-C1: Complete daydreamer/ClusterStability.ts

  • Detect high-variance volumes (unstable)
  • Trigger split (K-means with K=2)
  • Detect low-count volumes
  • Trigger merge with nearest neighbor volume
  • Re-run community detection and update PageActivity after split/merge

• P3-C2: Add cluster stability test coverage

  • tests/daydreamer/ClusterStability.test.ts (extend from P2-F)
  • Test split produces two balanced volumes
  • Test merge produces one combined volume
  • Test community labels updated after structural change

Exit Criteria: Clusters stay balanced over time; community labels stay current.