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.go

@@ -23,6 +23,7 @@ type Engine struct {
 	Degraded  []string // static: missing keys
 	TavilyCap int
 	Now       func() time.Time
+	NoRerank  map[string]bool
 
 	mu             sync.Mutex
 	Queries        map[string]map[string]uint64
@@ -137,6 +138,7 @@ func (e *Engine) Run(ctx context.Context, query string, k int, doRerank bool, ca
 	}
 	wg.Wait()
 
+	var lists []namedHits
 	var hits []Hit
 	var warns []string
 	var okNames, failNames, att []string
@@ -158,7 +160,7 @@ func (e *Engine) Run(ctx context.Context, query string, k int, doRerank bool, ca
 			continue
 		}
 		okNames = append(okNames, o.name)
-		hits = append(hits, o.out.Hits...)
+		lists = append(lists, namedHits{name: o.name, hits: o.out.Hits})
 		if o.out.Credits > 0 {
 			e.addCredits(caller, o.name, o.out.Credits)
 			if o.name == NameTavily && e.Cache != nil {
@@ -166,24 +168,9 @@ func (e *Engine) Run(ctx context.Context, query string, k int, doRerank bool, ca
 			}
 		}
 	}
-	hits = Dedupe(hits)
-	sortBySourceOrder(hits)
+	hits = mergeSearch(e, lists, doRerank)
 
 	var rerankMS int64
-	if doRerank && e.RerankOn {
-		rs := e.now()
-		ranked, fb := rankHits(ctx, query, hits, e.Embedder)
-		rerankMS = e.now().Sub(rs).Milliseconds()
-		if fb != "" {
-			warns = append(warns, "rerank fell back: "+fb)
-			degraded = true
-			e.mu.Lock()
-			e.RerankFallback++
-			e.mu.Unlock()
-		} else {
-			hits = ranked
-		}
-	}
 	hits = clampHits(hits, k)
 
 	if !degraded && e.Cache != nil && len(att) > 0 {