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/rank.go

@@ -3,6 +3,7 @@ package search
 import (
 	"context"
 	"math"
+	"sort"
 	"strings"
 )
 
@@ -79,3 +80,77 @@ func rankHits(ctx context.Context, query string, hits []Hit, emb Embedder) (out
 	}
 	return out, ""
 }
+
+const rrfK = 60
+
+type namedHits struct {
+	name string
+	hits []Hit
+}
+
+func rrfMerge(lists [][]Hit) []Hit {
+	type acc struct {
+		h Hit
+		s float64
+	}
+	seen := map[string]*acc{}
+	var order []string
+	for _, list := range lists {
+		for r, h := range list {
+			c := CanonicalURL(h.URL)
+			if c == "" {
+				continue
+			}
+			h.URL = c
+			sc := 1.0 / float64(rrfK+r+1)
+			if a, ok := seen[c]; ok {
+				a.s += sc
+				if len(h.Snippet) > len(a.h.Snippet) {
+					src := a.h.Source
+					a.h = h
+					a.h.Source = src
+				}
+				continue
+			}
+			cp := h
+			seen[c] = &acc{h: cp, s: sc}
+			order = append(order, c)
+		}
+	}
+	out := make([]Hit, 0, len(seen))
+	for _, c := range order {
+		a := seen[c]
+		a.h.Score = a.s
+		out = append(out, a.h)
+	}
+	sort.SliceStable(out, func(i, j int) bool {
+		return out[i].Score > out[j].Score
+	})
+	return out
+}
+
+func mergeSearch(e *Engine, lists []namedHits, doRerank bool) []Hit {
+	if doRerank && e != nil && e.RerankOn {
+		var hold, rest [][]Hit
+		for _, l := range lists {
+			if e.NoRerank[l.name] {
+				hold = append(hold, l.hits)
+			} else {
+				rest = append(rest, l.hits)
+			}
+		}
+		var out []Hit
+		for _, h := range hold {
+			out = append(out, h...)
+		}
+		out = append(out, rrfMerge(rest)...)
+		return Dedupe(out)
+	}
+	var all []Hit
+	for _, l := range lists {
+		all = append(all, l.hits...)
+	}
+	all = Dedupe(all)
+	sortBySourceOrder(all)
+	return all
+}