fix(attractor): correct inverted community hierarchy (#33)

[raphasouthall]

Summary

Fixes the inverted community hierarchy reported in #33, where vault_stats returned communities_coarse=12, communities_fine=8 (fine had fewer, larger basins than coarse).

Three compounding issues drove the inversion: (1) a shared TOP_K_NEIGHBORS=50 at both β levels funnelled notes through the same hub attractors at high β, (2) singleton merging absorbed the narrow 1-note basins that fine is supposed to expose, (3) only β differed between levels.

Changes

• Per-level top-K (TOP_K_COARSE=80, TOP_K_FINE=20) plumbed through _attractor_convergence so each pass uses an appropriate neighbourhood.
• Singleton-merge gate — _assign_communities(merge_singletons=bool); the fine pass calls it with False so narrow basins survive.
• Modularity + size stats — _modularity() computes weighted Newman Q on pre-sparsification S; detect_communities logs a warning if Q(fine) <= Q(coarse) so a future regression is visible at index time.
• Persistence — new community_level_stats table (schema v13 → v14) stores n, min/max/mean size, modularity per level.
• vault_stats output — adds community_levels: [...] array (legacy communities_coarse/fine/summarized preserved).

Results on production vault (539 notes)

Metric	Before (issue #33)	After
coarse n	12	10
fine n	8 (inverted)	13
fine min size	11	3
fine giants ≥100	2	1
n_fine > n_coarse	❌	✅

The modularity sanity warning fires honestly — Q(fine)=0.0527 < Q(coarse)=0.0628. The count hierarchy is correct but weighted modularity still favours coarse, a known property with heavy-tailed edge weights. Surfaced for future investigation rather than silently hidden.

Test plan

• 544 tests pass locally (uv run pytest tests/)
• Ruff clean on changed files
• 13 new test cases cover top_k override, merge gate, modularity (2-block / single-community / degenerate), size stats
• Schema migration v13→v14 idempotent (existing tests updated)
• Deployed to production LXC — service restart clean, vault_stats over MCP returns new block, vault_communities at both levels returns coherent results with narrow basins (3-note, 7-note) surviving at fine level

Notes

• Issue's proposed item 5 (rename coarse/fine → broad/narrow) deferred. The modularity warning will signal at index time if the hierarchy tuning fails to deliver on any given vault; revisit the rename only if the warning persists.

Closes #33