Boron Context 0.7.6 source-coverage eligibility

[kforris]

Summary

• add source-window eligibility contract v2 with measured numerator, eligible denominator, exclusions, unobservable evidence, and reason counts
• label legacy evidence additively via migration 012 and preserve historical mixed fields for compatibility
• instrument live Codebase Memory, Markdown, and HTTP source sizes; surface 7/30-day metrics in MCP/API/Inspector/menu bar

Verification

• npm run check (64 passed, 12 environment-skipped; held-out 6 cases, recall@5/MRR/source coverage 1.00; wrong-project retrieval/writeback 0)
• isolated PostgreSQL migration + 12/12 integration tests; migration 012 idempotency/schema verified
• swift test (5/5) and swift build -c release
• npm run format:check
• npm audit --omit=dev --audit-level=high (0 vulnerabilities)
• npm pack --dry-run --json

Boundary

This is the single 0.7.6 productization milestone. It does not publish a release, submit to Marketplace, spend money, or start marketing.