fix(fb-37): address PR #129 round 1 review

- doc TraceFusionInput.fusion_score semantics (single-mode vs hybrid)
- comment why total_ms vs stage sum can drift (millis truncation)
- TODO marker on TUI trace popup filter passthrough

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-search/src/hybrid.rs

@@ -391,6 +391,10 @@ impl HybridRetriever {
             }
         }
 
+        // total_ms is wall-clock from start; per-stage `lexical_ms` /
+        // `vector_ms` / `fusion_ms` each truncate to whole millis via
+        // `as_millis() as u64`, so their sum can drift below total
+        // (sub-ms losses) — DO NOT assert `total_ms >= sum(stages)`.
         tb.timing.total_ms = start_total.elapsed().as_millis() as u64;
         Ok((final_hits, tb.into_trace()))
     }