Wait for the stall-watchdog test child to close before removing its database

The WriteBufferManager stall-watchdog test can fail during temporary database cleanup on Bun/Windows because `runStallChild()` resolves as soon as it sends `SIGKILL`, before observing the child's `close` event. The caller immediately removes the database directory, potentially racing the still-running child and its open handles. A cleanup exception also masks the watchdog assertions that follow the `finally` block.

Observed on Bun 1.4.2, Windows x64, RocksDB 11.8.1 in [PR #862's CI job](https://github.com/HarperFast/rocksdb-js/actions/runs/35178804535/job/105066312389), at head `b2825a1a346bd603d86d1474bb263182a5c40406` (merge ref `4a09254`). The failing case was `WriteBufferManager stall watchdog > keeps reporting while shutdown waits for a stalled writer`, with `EBUSY: resource busy or locked, rm` at `test/write-buffer-manager-stall-watchdog.test.ts:216`. The failure persisted past the existing `rmSync` retry options. This is a confirmed cleanup failure; the exact Windows handle responsible was not captured.

The ordering defect already exists in [main at b4d1045, runStallChild](https://github.com/HarperFast/rocksdb-js/blob/b4d104562e7d5353b6d7a4412e0a8b48ee805062/test/write-buffer-manager-stall-watchdog.test.ts#L34): `finish()` sends the kill signal and resolves immediately. The `close` event is wired to the same already-settled function, so callers do not wait for it. The adjacent `runExitChild()` helper resolves from `close`, which is the intended pattern. PR #862 does not modify this test, and its latest commit changes comments only; no main CI reproduction is claimed.

Suggested fix: separate requesting termination from completing the result, resolve only after child close/reaping, and retain a bounded hard deadline and useful captured output for failure diagnostics. Do not merely lengthen deletion retries or suppress cleanup errors. Verify success, deadline, and spawn-error paths; run the test on Bun/Windows so the directory removal is exercised after child closure.

The same job's first suite attempt also reported an unexpected Vitest fork-worker exit. Its cause is unknown and is not attributed to this cleanup race. The cleanup error occurred on the workflow's built-in second attempt. A targeted CI-job rerun has been requested.

Priority: P2 — a contained, visible CI/test-lifecycle defect that can mask the real assertion and leave a child or temporary database behind; no production data loss or customer incident is established. No release milestone or owning epic is currently identified. No customer or security context applies.

Start in test/write-buffer-manager-stall-watchdog.test.ts at runStallChild() and compare it with the adjacent runExitChild() helper. Run the targeted “WriteBufferManager stall watchdog” test, then verify success, deadline, and spawn-error paths. Done means the child is closed before database removal, useful output is retained, and Bun/Windows no longer produces EBUSY or masks watchdog assertions.

Written by the indexing model from the issue text.