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)
+	}
 }