fix(insights): replace density threshold with mean intra-degree

[HUAN2022A]

Fixes #731

Problem

On large wikis the Sparse cluster knowledge-gap warning fires on nearly every community. The reporter's math is correct and confirmed against the code: cohesion is the classic density 2E / (n(n-1)), which decays as O(1/n). With the fixed threshold cohesion < 0.15, no community larger than ~55 pages (at the reporter's mean degree of 8.1) can ever pass, so real, well-linked topic clusters are permanently flagged — and the suggested remedy ("add links") pushes users toward filler wikilinks.

Fix (reporter's option 1 — scale-invariant)

• CommunityInfo gains meanIntraDegree = 2 * intraEdges / nodeCount (average links per page into the same community), computed alongside the existing cohesion in detectCommunities (cohesion itself is kept — it is asserted in existing tests and remains meaningful).
• New shared constant SPARSE_CLUSTER_MIN_MEAN_INTRA_DEGREE = 2 in wiki-graph.ts, consumed by both flag sites so they cannot drift apart:
  • detectKnowledgeGaps (graph-insights.ts): condition becomes meanIntraDegree < 2 && nodeCount >= 3, and the description now states something actionable: "N pages average X.X links to other pages in this cluster".
  • the community panel "!" marker in graph-view.tsx, which had a second, independent hard-coded 0.15.

The reporter's own numbers validate the boundary: at mean degree 8.1, a 55-page community scores cohesion exactly 0.15 (n=56 drops below) — precisely the "~55 pages max" they measured.

Testing

• wiki-graph-analysis.test.ts: two 4-cliques + a bridge → each community meanIntraDegree === 3, plus a nodeCount-weighted conservation assertion (Σ n·meanIntraDegree === 2 × intra-community edges) that holds for any Louvain partition.
• graph-insights.test.ts (new): a 500-node community with mean intra-degree 8 (cohesion 0.016) is not flagged (previously always flagged); a 10-node community at 0.8 is flagged with the new description; boundary at exactly 2.0 not flagged; <3 nodes not flagged; isolated/bridge paths unregressed.
• npm run test:mocks: 133 files / 1883 tests passed · npm run typecheck: clean