[dailyun]

Async iteration never finishes when neither stdout nor stderr is piped, even after the child exits. For example:

const proc = x(process.execPath, ['-e', ''], {
  nodeOptions: { stdio: 'ignore' }
});
for await (const line of proc) console.log(line);
console.log('finished'); // never reached before this fix

combineStreams([]) creates a stream that nobody ends. End it immediately when there are no inputs. The iterator still waits for the child to close, so throwOnError continues to report nonzero exits. This also fixes stdio: 'inherit' without changing output capture.

All four new regression cases time out on the previous code and pass with the fix. Full suite: 55 passed, 10 skipped on Node 24.19.0 and via the Bun 1.4.3 build/test command. Build, lint (including TypeScript and publint), and formatting pass.

Implemented and tested with Codex assistance.