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-core/src/search.rs

@@ -158,6 +158,9 @@ pub struct TraceFusionInput {
     pub chunk_id: ChunkId,
     pub lexical_rank: Option<u32>,
     pub vector_rank: Option<u32>,
+    /// Hybrid mode: normalized RRF score in `[0, 1]`.
+    /// Lexical / Vector mode: equals the underlying retriever's score
+    /// (no fusion ran). 0.0 for chunks dropped past `target_k`.
     pub fusion_score: f32,
 }
 

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()))
     }

crates/kebab-tui/src/search.rs

@@ -227,6 +227,8 @@ pub fn handle_key_search(state: &mut App, key: KeyEvent) -> KeyOutcome {
             return KeyOutcome::Continue;
         }
         if let Some((q_text, q_mode)) = last_query {
+            // TODO: thread filters when TUI gains a filter UI (currently
+            // mirrors fire_search which also passes default filters).
             let q = kebab_core::SearchQuery {
                 text: q_text,
                 mode: q_mode,