p3: keep original URLs on RRF/dedupe; rerank=false is not a merge

CanonicalURL is the dedupe key only. Hits keep the first source's
original URL, including a trailing slash. Dedupe has the same rule.

internal/search/engine_test.go

@@ -157,6 +157,23 @@ func TestRRFInterleaveAndNoRerank(t *testing.T) {
 	}
 }
 
+func TestRRFKeepsOriginalURL(t *testing.T) {
+	orig := "https://ex.com/docs/"
+	merged := rrfMerge([][]Hit{
+		{{URL: orig, Title: "slash", Source: NameSearxNG}},
+		{{URL: "https://ex.com/docs", Title: "stripped", Snippet: "longer", Source: NameTavily}},
+	})
+	if len(merged) != 1 {
+		t.Fatalf("%+v", merged)
+	}
+	if merged[0].URL != orig {
+		t.Fatalf("want original URL %q got %q", orig, merged[0].URL)
+	}
+	if CanonicalURL(orig) != "https://ex.com/docs" {
+		t.Fatalf("canonical %q", CanonicalURL(orig))
+	}
+}
+
 func TestDailyCapSkipsTavily(t *testing.T) {
 	db, err := dcache.Open(filepath.Join(t.TempDir(), "c.db"), 1<<20, nil)
 	if err != nil {
@@ -187,4 +204,7 @@ func TestDedupeLongerSnippet(t *testing.T) {
 	if len(out) != 1 || out[0].Source != NameSearxNG || out[0].Snippet != "aabbcc" {
 		t.Fatalf("%+v", out)
 	}
+	if out[0].URL != "https://Ex.com/a#x" {
+		t.Fatalf("dedupe must keep first original URL, got %q", out[0].URL)
+	}
 }

internal/search/rank.go

@@ -97,18 +97,20 @@ func rrfMerge(lists [][]Hit) []Hit {
 	var order []string
 	for _, list := range lists {
 		for r, h := range list {
-			c := CanonicalURL(h.URL)
+			orig := h.URL
+			c := CanonicalURL(orig)
 			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
+					keep := a.h.URL
 					a.h = h
 					a.h.Source = src
+					a.h.URL = keep
 				}
 				continue
 			}

internal/search/types.go

@@ -79,13 +79,14 @@ func Dedupe(in []Hit) []Hit {
 		if canon == "" {
 			continue
 		}
-		h.URL = canon
 		if s, ok := seen[canon]; ok {
 			if len(h.Snippet) > len(s.h.Snippet) {
 				src := s.h.Source
+				keep := s.h.URL
 				sc := s.h.Score
 				s.h = h
 				s.h.Source = src
+				s.h.URL = keep
 				if sc > s.h.Score {
 					s.h.Score = sc
 				}