feat: add durable evidence accumulator

[imran-siddique]

Summary

• add bounded per-run evidence accumulation before operational sampling
• add monotonic sequencing and a documented deterministic SHA-256 chain profile
• support memory and acknowledged durable-callback modes with fail-closed projection
• isolate retained evidence from caller and callback mutation

Evidence

• focused evidence tests: 10 passed
• full suite: 35 passed, 15 subtests passed
• schema, version, conformance, and compile gates pass
• wheel build and fresh-environment installation pass

Limits

• memory mode is development-only
• adopter owns callback storage, idempotency, and recovery
• completeness is caller-assessed; TRACE mapping is not part of this PR