feat: 3-lane RRF — graph agent as third search lane

Graph lane expands seed results from semantic + FTS by following
wikilinks. Weight 0.8 (slightly below direct lanes). Detail field
on LaneContribution shows hop info in --explain output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: devwhodevs

src/fusion.rs

@@ -6,6 +6,7 @@
 ///   rrf_score = sum( weight_i / (k + rank_i) )
 ///
 /// A ranked result from a single search lane.
+#[derive(Clone)]
 pub struct RankedResult {
     pub file_path: String,
     pub file_id: i64,
@@ -32,6 +33,7 @@ pub struct LaneContribution {
     pub rank: usize,
     pub raw_score: f64,
     pub weighted_contribution: f64,
+    pub detail: Option<String>, // e.g., "1-hop from BRE-2579"
 }
 
 use std::collections::HashMap;
@@ -93,6 +95,7 @@ pub fn rrf_fuse(lanes: &[(&str, &[RankedResult], f64)], k: usize) -> Vec<FusedRe
                 rank,
                 raw_score: r.score,
                 weighted_contribution: contribution,
+                detail: None,
             });
         }
     }
@@ -124,10 +127,15 @@ pub fn rrf_fuse(lanes: &[(&str, &[RankedResult], f64)], k: usize) -> Vec<FusedRe
 pub fn format_explain(result: &FusedResult) -> String {
     let mut out = format!("  RRF: {:.4}\n", result.rrf_score);
     for lc in &result.lane_contributions {
-        out.push_str(&format!(
-            "    {}: rank #{}, raw {:.2}, +{:.4}\n",
-            lc.lane_name, lc.rank, lc.raw_score, lc.weighted_contribution,
-        ));
+        let detail_str = lc
+            .detail
+            .as_deref()
+            .map(|d| format!(" ({})", d))
+            .unwrap_or_default();
+        out += &format!(
+            "    {}: rank #{}, raw {:.2}{}, +{:.4}\n",
+            lc.lane_name, lc.rank, lc.raw_score, detail_str, lc.weighted_contribution
+        );
     }
     out
 }
@@ -234,12 +242,14 @@ mod tests {
                     rank: 1,
                     raw_score: 0.87,
                     weighted_contribution: 0.0164,
+                    detail: None,
                 },
                 LaneContribution {
                     lane_name: "fts".to_string(),
                     rank: 3,
                     raw_score: 5.23,
                     weighted_contribution: 0.0159,
+                    detail: None,
                 },
             ],
         };

src/search.rs

@@ -6,6 +6,7 @@ use serde_json::json;
 
 use crate::embedder::Embedder;
 use crate::fusion::{self, RankedResult};
+use crate::graph;
 use crate::hnsw::HnswIndex;
 use crate::store::{Store, StoreStats};
 
@@ -127,12 +128,31 @@ pub fn run_search(
             .unwrap_or(std::cmp::Ordering::Equal)
     });
 
+    // --- Graph lane ---
+    // Combine seeds from semantic + FTS (deduplicated by file_path, take higher score)
+    let combined_seeds: Vec<RankedResult> = {
+        let mut by_file: HashMap<String, RankedResult> = HashMap::new();
+        for r in semantic_results.iter().chain(fts_results.iter()) {
+            match by_file.get(&r.file_path) {
+                Some(existing) if r.score <= existing.score => {}
+                _ => {
+                    by_file.insert(r.file_path.clone(), r.clone());
+                }
+            }
+        }
+        by_file.into_values().collect()
+    };
+
+    let graph_results =
+        graph::graph_expand(&store, &combined_seeds, query, 2, 20).unwrap_or_default();
+
     // --- RRF Fusion ---
     const RRF_K: usize = 60;
     let fused = fusion::rrf_fuse(
         &[
             ("semantic", &semantic_results, 1.0),
             ("fts", &fts_results, 1.0),
+            ("graph", &graph_results, 0.8),
         ],
         RRF_K,
     );