G1: Recall 管线完整化 — 搜索缓存 + 双向 BFS 导航

改动：
- G1.3: graph/navigate API 新增 source+target 参数，调用 NavigateBiDir 双向 BFS
- G1.4: recall pipeline 新增 Step 0 搜索缓存检查 + Step 6.5 缓存写入
  缓存 TTL 1h，相同 query+namespace 跳过完整管线直接返回
- MMR 多样性去重（G1.1）+ 多跳图谱扩展（G1.2）已在上次提交中存在

测试：双向 BFS 返回 9 条路径，bidirectional=true；缓存命中测试通过

go/internal/api/routes/graph.go

@@ -65,9 +65,14 @@ func (ga *GraphAPI) Query(w http.ResponseWriter, r *http.Request) {
 }
 
 // POST /api/v1/graph/navigate
+// 支持两种模式：
+//   1. 单实体 BFS: {"entity": "...", "max_hops": 2}
+//   2. 双向 BFS: {"source": "...", "target": "...", "max_hops": 3}（设计文档 §2.5.4）
 func (ga *GraphAPI) Navigate(w http.ResponseWriter, r *http.Request) {
 	var req struct {
 		Entity    string `json:"entity"`
+		Source    string `json:"source"`
+		Target    string `json:"target"`
 		MaxHops   int    `json:"max_hops"`
 		Namespace string `json:"namespace"`
 	}
@@ -75,14 +80,31 @@ func (ga *GraphAPI) Navigate(w http.ResponseWriter, r *http.Request) {
 		respondError(w, 400, "invalid body")
 		return
 	}
-	if req.Entity == "" {
-		respondError(w, 400, "entity required")
-		return
-	}
 	if req.MaxHops <= 0 {
 		req.MaxHops = 2
 	}
 
+	// 模式 2: 双向 BFS（source + target）
+	if req.Source != "" && req.Target != "" {
+		source := normalizeEntity(req.Source)
+		target := normalizeEntity(req.Target)
+		paths, err := ga.Graph.NavigateBiDir(source, target, req.MaxHops, req.Namespace)
+		if err != nil {
+			respondError(w, 500, "navigate failed: "+err.Error())
+			return
+		}
+		respond(w, 200, map[string]interface{}{
+			"paths": paths, "source": req.Source, "target": req.Target,
+			"bidirectional": true, "count": len(paths),
+		})
+		return
+	}
+
+	// 模式 1: 单实体 BFS（兼容旧格式）
+	if req.Entity == "" {
+		respondError(w, 400, "entity (or source+target) required")
+		return
+	}
 	entity := normalizeEntity(req.Entity)
 	paths, err := ga.Graph.Navigate(entity, req.MaxHops, req.Namespace)
 	if err != nil {

go/internal/storage/recall.go

@@ -2,6 +2,7 @@
 package storage
 
 import (
+	"encoding/json"
 	"fmt"
 	"math"
 	"time"
@@ -50,6 +51,16 @@ func (p *RecallPipeline) Recall(query, namespace string, topK int, diversity flo
 		topK = 10
 	}
 
+	// Step 0: 搜索缓存（§2.6 — 相同 query hash → TTL 1h → 命中直接返回）
+	if SearchCacheInstance != nil {
+		if cached, ok := SearchCacheInstance.Get(query, namespace); ok {
+			var results []models.RecallResult
+			if err := json.Unmarshal(cached, &results); err == nil {
+				return results, nil
+			}
+		}
+	}
+
 	// Step 1: Encode query
 	queryVec, err := p.embedder.EncodeSingle(query)
 	if err != nil {
@@ -158,6 +169,11 @@ func (p *RecallPipeline) Recall(query, namespace string, topK int, diversity flo
 	}
 	results = deduped
 
+	// Step 6.5: 写搜索缓存（§2.6 — TTL 1h，下次相同 query 直接返回）
+	if SearchCacheInstance != nil {
+		SearchCacheInstance.Set(query, namespace, results)
+	}
+
 	// Step 7: 预取推送（CO_OCCURS 权重 > 0.6 的配套记忆 → WebSocket）
 	if p.prefetch != nil {
 		// 使用 CO_OCCURS 追踪器收集真实预取候选项