chore(scale): add content-cache invalidation keyed by canonical graph hash

[parthrohit22]

Parent: #210

Goal

Add a bounded content-cache layer that cannot serve stale analysis after a revision changes.

Scope

• Key cache entries by the existing canonical graph/hash identity.
• Invalidate or bypass entries when the sealed revision or relevant inputs change.
• Keep cache behavior deterministic across worker restarts.
• Avoid caching secrets, source contents outside the existing sealed-storage boundary, or cross-owner data.

Acceptance criteria

• Same canonical input reuses a valid cache entry.
• Revision or input changes invalidate the old entry.
• Cache entries are owner-scoped and bounded.
• Worker restart does not corrupt cache state.
• Tests cover hit, miss, invalidation, eviction, and cross-owner isolation.