feat: finish detector and parallel scan backlog (#258, #263, #266)

[ColumbusLabs]

Summary

Completes the remaining detector and performance backlog:

• verifies and documents floating-promise confidence behavior (Add floating / unawaited promise detector #258)
• completes configurable, opt-in stale feature-flag analysis with conservative registry receiver scoping (Add stale feature-flag debt detector #263)
• adds real worker-thread scans, explicit cross-file aggregation, portable shared cache v3, RFC/docs, and a correctness/speedup benchmark (Parallel worker-thread scanning + shareable scan cache #266)

Verification

• npm run test:all (925/925)
• npm run build
• schema generation drift check
• parallel benchmark: byte-identical findings and >=1.05x median speedup
• full CLI self-scan with --concurrency 4
• independent final review: complete, no findings

Closes #263
Closes #266

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