[security] Chunk/prefix cache keys ignore source and policy #83

Problem

Chunk and safe-prefix cache keys are content-hash (plus document / normalizer / model version) only. They do not include source/trust level, scan policy, or scanner set. A result cached under a permissive path (e.g. USER / ALLOW) can be reused for a stricter path (e.g. RETRIEVED), bypassing intended taint and policy differences.

Expected / acceptance criteria

• Cache key includes source (and/or trust level), effective policy identity, and scanner set (or equivalent fingerprint)
• Same text under different source/policy/scanner config does not reuse a prior chunk or prefix entry
• Regression: USER allow must not satisfy a later RETRIEVED (or stricter) scan via cache hit

Related

Safe-prefix boundary overlap is tracked separately.