Implements async shadow traffic to staging clusters for comparison:
- Completes TODOs in shadow.rs: compute symmetric diff (hit IDs only in shadow)
- Adds admin API endpoints: GET /_miroir/shadow/diff, GET /_miroir/shadow/stats
- Adds shadow_manager to AppState for admin endpoint access
- Adds acceptance tests: 5% sampling rate, ring buffer bounds, operations filter
Key features:
- Stateles per-request scaling via local RNG
- Shadow failures never impact primary (timeout budget enforced)
- Ring buffer evicts oldest when full (in-memory only, per plan §4)
- Only search/multi_search/explain operations shadowed (writes excluded)
Acceptance criteria met:
- 5% sampling rate verified in test (±2% tolerance over 10K queries)
- Ring buffer bounded and evicts oldest entries
- Operations filter enforces write exclusion
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Closes: miroir-uhj.16
62e5df369f