feat(subscribe): wrap yields in ResponseEnvelope

- Change subscribe() return type to AsyncGenerator<ResponseEnvelope, void, unknown>
- Check isResponseEnvelope() on each yielded value: pass through if already an envelope
- Wrap raw values with localEnvelope(value, operationId) with fresh timestamp per yield
- Preserve generator cleanup (generator.return()) in finally block
- Preserve existing spec/handler not-found error behavior
- Add 13 tests covering wrapping, passthrough, timestamps, mixed yields, early termination

src/subscribe.ts

@@ -1,12 +1,13 @@
 import type { OperationContext } from "./types.js";
 import { OperationRegistry } from "./registry.js";
+import { type ResponseEnvelope, isResponseEnvelope, localEnvelope } from "./response-envelope.js";
 
 export async function* subscribe(
   registry: OperationRegistry,
   operationId: string,
   input: unknown,
   context: OperationContext,
-): AsyncGenerator<unknown, void, unknown> {
+): AsyncGenerator<ResponseEnvelope, void, unknown> {
   const spec = registry.getSpec(operationId);
 
   if (!spec) {
@@ -23,7 +24,11 @@ export async function* subscribe(
 
   try {
     for await (const value of generator) {
-      yield value;
+      if (isResponseEnvelope(value)) {
+        yield value;
+      } else {
+        yield localEnvelope(value, operationId);
+      }
     }
   } finally {
     if (generator.return) {