Fix 8 semantic bugs in a hand-rolled Node.js event emitter (re-entrancy, once/off, error events, leak warning)

# Task: fix the plugin host's event emitter `emitter.js` (Node 20, CommonJS, no dependencies) has several bugs reported by plugin authors. Fix them so it behaves as follows: 1. `emit` calls the listeners registered at the moment `emit` starts, in registration order, even if listeners add or remove listeners during the emit (removals/additions affect only later emits). 2. `once(ev, fn)` listeners run at most once even if the event is emitted again from inside the listener (re-entrant emit), and `off(ev, fn)` with the original `fn` removes a pending `once` listener. 3. `emit` returns `true` if there was at least one listener, else `false` (also when the listener list exists but is empty). 4. Emitting `'error'` with no `'error'` listener throws the error argument (or `new Error('Unhandled error')` if none was given); with a listener it is just delivered. 5. A listener that throws stops the emit and the error propagates to the `emit` caller, but `once` listeners that threw are still removed. 6. The leak warning is printed at most once per event name, when the count first exceeds the max; `setMaxListeners(0)` disables it. 7. `off` removes only the most recently added matching registration when the same function was added twice. 8. `on`, `once`, `off`, `setMaxListeners` return `this`; `listenerCount` counts pending once listeners. Keep the export shape. Add your own tests under `test/` if you want (`node --test test/`). No network, no npm installs.