domain: route fs callback throws to uncaughtException, unblocking 4 tests (domain 88%→96%) - #34661

The 4b143eb72d `guardCallback` sweep wrapped `internal/fs/watch.ts:162` and `internal/fs/watchfile.ts:26`, but this third native `fs.watch` listener consumer passes an unwrapped callback that invokes the user-supplied `ignoreMatcher?.(filename)` at L106. If `options.ignore` is a function that throws, the throw reaches native `emit_js` → `report_active_exception_as_unhandled` → keep-alive `uncaught_exception()`, and with this PR's removal of `unhandled_error_counter` from `is_event_loop_alive_exc

The `once()` override is now a pure passthrough after 8eb8f030a9 collapsed `#onOrOnce` into `#armAndOn`: `super.once()` (events.ts:343) does `this.on(type, _onceWrap(...))`, which dispatches to BunWebSocket's overridden `on()` → `#armAndOn()`, so inheriting `EventEmitter.prototype.once` directly is byte-identical. Per REVIEW.md § *Delete dead code in the same PR that makes it dead*, drop the three-line override (the `onceObject` constant is still used at L312/L548 and stays).

The bare `try { this.emit('error', ...) } catch {}` in the new `'abort'` branch is broader than its stated purpose (suppressing ERR_UNHANDLED_ERROR when no listener exists): it also swallows a throw from the user's own `'error'` listener, whereas the sibling `'error'` branch at L199 emits unwrapped so a throwing listener there reaches `guardCallback` → `reportUncaughtException` → fatal. Gate on `if (this.listenerCount('error') > 0) this.emit('error', filenameOrError)` instead — that keeps `test-