feat: metrics snapshot test harness for scenario verification

- MetricsSnapshot::capture() snapshots all metric values
- assert_counter_inc(): verify counter delta after scenario
- assert_gauge_eq(): verify gauge value
- assert_histogram_count_inc(): verify histogram observations
- assert_gauge_f64_approx(): verify f64 gauges with tolerance
- print_deltas(): debug helper for all changed metrics
- 9 scenario tests: ingest, query error, relevance batch, write
- Histogram fields made pub for snapshot access
- 515 total tests passing

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 metrics_snapshot;
 pub mod relevance_judge;
 pub mod query;
 pub mod auth;