perf(scan): cache unchanged objective scan results

[DivyamTalwar]

Summary

• add a persistent content-addressed cache for objective and CI scan profiles
• invalidate on source stat changes, relevant configuration, selected phases/profile, tool version, and zone overrides
• fail open on corrupt cache state and keep full subjective scans uncached

Verification

• pytest -q tests/scan/test_incremental_cache.py: 3 passed
• ruff check on changed Python files: passed
• 10,000-file benchmark: cold 0.6434s, warm 0.5673s; warm scan reports a cache hit and skips detector phases

Stack

This PR targets #59's branch because repository settings currently disallow the required merge-commit method on #59. Retarget to main after #59 is merged.