- 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
