refactor(mesh-brain): replace graspologic Leiden with NetworkX Louvain — unblock numpy 2.x + py3.13 (#10524)

[mrveiss]

Thinking Path

Part of #10524 (Task 1). The repo can't unify on a single latest numpy because graspologic 3.4.4 (latest, no numpy-2 release even in dev) hard-pins numpy<2.0.0 and requires_python <3.13 — silently capping the backend a major version behind and blocking the py3.14 migration. graspologic was used in exactly one place: two leiden(G, trials=N) calls in community_clusterer.py. NetworkX (already a dependency) ships weight-aware Louvain with the same {node: community_id} shape, so the swap removes the blocker without adding a package.

What Changed

• community_clusterer.py — new _detect_communities() helper using networkx.algorithms.community.louvain_communities (seeded for determinism); both Leiden call sites now route through it; graspologic imports removed. Return contract unchanged.
• requirements.txt — removed graspologic>=3.4.4; lifted the opencv-python-headless<4.11 cap (numpy>=2 is now resolvable in the backend).
• lifespan.py — removed the dead except ImportError → "pip install graspologic" → sleep 24h branch. NetworkX is a hard dependency, so that condition can no longer occur; the generic except Exception still handles real failures.
• tests — removed the graspologic sys.modules stubs from community_clusterer_test.py and mesh_db_adapter_test.py, plus the two tests asserting the now-removed ImportError/24h-pause behavior. All clustering assertions retained and now run against real Louvain.

Verification

$ python3 -m pytest services/mesh_brain/community_clusterer_test.py \
                     services/mesh_brain/mesh_db_adapter_test.py -q
31 passed in 0.89s

• Triangle → 1 community → 1 centroid; two disconnected triangles → 2 communities → 2 centroids — both hold under real Louvain.
• grep -rn graspologic --include=*.py → only explanatory comments remain (no code refs).
• py_compile clean on all changed files.

Follow-ups (Task 2/3 of #10524): add constraints/shared.txt as the single numpy source of truth referenced by every component; CI guard against version re-drift across portable/co-locatable roles.

Model Used

Claude Opus 4.8 (1M context)

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!