feat(O4): relevance judge with Prometheus metrics

- RelevanceJudge: threshold-based relevance evaluation
- evaluate(): single query-result pair scoring
- evaluate_batch(): batch eval with precision/recall/F1
- Tracks: evals total, relevant/irrelevant, score histogram
- Updates precision/recall/F1 gauges per batch
- 4 unit tests passing
- Metrics: R1-R9 (9 metrics instrumented)

crates/mem-cli/src/lib.rs

@@ -2,6 +2,7 @@ pub mod endpoints;
 pub mod handlers;
 pub mod http_server;
 pub mod metrics;
+pub mod relevance_judge;
 pub mod query;
 pub mod auth;
 pub mod ingest_worker;