Add WithNotFoundHandler option for custom 404 responses on Router

Allows configuring a custom handler for unmatched routes, enabling
consistent JSON error responses instead of ServeMux's default plain
text. NewRouter now accepts RouterOption functional options.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/route_notfound_test.go

@@ -0,0 +1,70 @@
+package server_test
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx/server"
+)
+
+func TestRouter_NotFoundHandler(t *testing.T) {
+	t.Run("custom 404 handler", func(t *testing.T) {
+		notFound := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusNotFound)
+			json.NewEncoder(w).Encode(map[string]string{"error": "not found"})
+		})
+
+		r := server.NewRouter(server.WithNotFoundHandler(notFound))
+		r.HandleFunc("GET /exists", func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})
+
+		// Matched route works normally.
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/exists", nil)
+		r.ServeHTTP(w, req)
+
+		if w.Code != http.StatusOK {
+			t.Fatalf("matched route: got status %d, want %d", w.Code, http.StatusOK)
+		}
+
+		// Unmatched route uses custom handler.
+		w = httptest.NewRecorder()
+		req = httptest.NewRequest(http.MethodGet, "/nope", nil)
+		r.ServeHTTP(w, req)
+
+		if w.Code != http.StatusNotFound {
+			t.Fatalf("not found: got status %d, want %d", w.Code, http.StatusNotFound)
+		}
+		if ct := w.Header().Get("Content-Type"); ct != "application/json" {
+			t.Fatalf("Content-Type = %q, want %q", ct, "application/json")
+		}
+
+		var body map[string]string
+		if err := json.Unmarshal(w.Body.Bytes(), &body); err != nil {
+			t.Fatalf("unmarshal: %v", err)
+		}
+		if body["error"] != "not found" {
+			t.Fatalf("error = %q, want %q", body["error"], "not found")
+		}
+	})
+
+	t.Run("default behavior without custom handler", func(t *testing.T) {
+		r := server.NewRouter()
+		r.HandleFunc("GET /exists", func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})
+
+		w := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/nope", nil)
+		r.ServeHTTP(w, req)
+
+		// Default ServeMux returns 404.
+		if w.Code != http.StatusNotFound {
+			t.Fatalf("got status %d, want %d", w.Code, http.StatusNotFound)
+		}
+	})
+}