feat: Support reuse of nested router

packages/dart_frog/lib/src/context.dart

@@ -42,4 +42,14 @@ This can happen if $T was not provided to the request context:
     }
     return (value as T Function())();
   }
+
+  /// Get URL parameters captured by the [Router.mount].
+  /// They can be accessed from inside the mounted routes.
+  Map<String, String> get mountedParams {
+    final p = request._request.context['dart_frog/mountedParams'];
+    if (p is Map<String, String>) {
+      return UnmodifiableMapView(p);
+    }
+    return _emptyParams;
+  }
 }

packages/dart_frog/lib/src/router.dart

@@ -142,7 +142,18 @@ class Router {
       effectivePath = urlPath;
     }
     final modifiedRequestContext = RequestContext._(
-      request._request.change(path: effectivePath),
+      request._request.change(
+        path: effectivePath,
+        context: {
+          // Include the parameters captured here as mounted parameters.
+          // We also include previous mounted params in case there is double
+          // nesting of `mount`s
+          'dart_frog/mountedParams': {
+            ...context.mountedParams,
+            ...params,
+          },
+        },
+      ),
     );
 
     return await Function.apply(handler, [

packages/dart_frog/test/src/router_test.dart

@@ -624,6 +624,44 @@ void main() {
     expect(catchAllResponse.body, equals('catch-all-handler'));
   });
 
+  test('dynamic routes mountedParams', () async {
+    final context = _MockRequestContext();
+
+    final usersRouter = () {
+      final router = Router();
+
+      String getUser(RequestContext c) => c.mountedParams['user']!;
+
+      router.get(
+        '/self',
+        (RequestContext context) => Response(body: "I'm ${getUser(context)}"),
+      );
+      return router;
+    }();
+
+    final app = Router()
+      ..mount('/users/<user>', (
+        RequestContext context,
+        String user,
+      ) {
+        return usersRouter(context);
+      });
+
+    server.mount((request) async {
+      when(() => context.request).thenReturn(
+        Request(request.method, request.requestedUri),
+      );
+      final response = await app.call(context);
+      final body = await response.body();
+      return shelf.Response(response.statusCode, body: body);
+    });
+
+    final response = await http.get(
+      Uri.parse('${server.url}/users/jack/self'),
+    );
+    expect(response.body, equals("I'm jack"));
+  });
+
   group('RouterEntry', () {
     void testPattern(
       String pattern, {