p3: map Brave LLM Context; RRF merge; per-source no_rerank

Live llm/context hits live in grounding.generic plus a URL-keyed
sources map, not a sources array. Fan-in uses reciprocal-rank fusion
instead of cosine. no_rerank keeps that source's own order as a prefix.

internal/search/engine_test.go

@@ -121,8 +121,39 @@ func TestRerankFallbackStill200(t *testing.T) {
 	if len(r.Hits) != 1 {
 		t.Fatal(r.Hits)
 	}
-	if !strings.Contains(strings.Join(r.Warnings, " "), "rerank fell back") {
-		t.Fatalf("%v", r.Warnings)
+	if strings.Contains(strings.Join(r.Warnings, " "), "rerank fell back") {
+		t.Fatalf("RRF must not depend on embed: %v", r.Warnings)
+	}
+}
+
+func TestRRFInterleaveAndNoRerank(t *testing.T) {
+	searx := []Hit{
+		{URL: "https://a.example/s", Title: "s1", Source: NameSearxNG},
+		{URL: "https://b.example/s", Title: "s2", Source: NameSearxNG},
+	}
+	tav := []Hit{
+		{URL: "https://tavily.example/1", Title: "t1", Source: NameTavily},
+		{URL: "https://a.example/s", Title: "t-dup", Snippet: "longer tavily snippet here", Source: NameTavily},
+	}
+	merged := rrfMerge([][]Hit{searx, tav})
+	if len(merged) != 3 {
+		t.Fatalf("%+v", merged)
+	}
+	if merged[0].URL != "https://a.example/s" {
+		t.Fatalf("shared URL should rank first, got %+v", merged[0])
+	}
+	e := &Engine{
+		Sources: []Source{
+			fakeSrc{name: NameSearxNG, hits: searx},
+			fakeSrc{name: NameTavily, hits: tav},
+		},
+		DefaultK: 8,
+		RerankOn: true,
+		NoRerank: map[string]bool{NameTavily: true},
+	}
+	r := e.Run(context.Background(), "q", 8, true, "titan", dcache.ClassFleet)
+	if r.Hits[0].URL != "https://tavily.example/1" {
+		t.Fatalf("no_rerank tavily should lead: %+v", r.Hits)
 	}
 }