Fix type assertion in protocol decode test

Use 'as const' on the type field to ensure the literal type "join" is
preserved, matching the ClientMessage union type requirements.

src/protocol.test.ts

@@ -9,7 +9,7 @@ describe("protocol", () => {
   });
 
   test("decode parses valid json", () => {
-    const msg = { type: "join", room: "test" };
+    const msg = { type: "join" as const, room: "test" };
     const decoded = decode(JSON.stringify(msg));
     expect(decoded).toEqual(msg);
   });