test(memory-core): gate the takeover on the lock oracle and make fixture teardown fail loud

- Recovery starts only after isLockOwnerProvenDead proves the SIGKILLed holder
  dead (bounded, identity-guarded, fails loud), not on the parent's exit event.
- A child that closes before printing the awaited line rejects its waiters
  with its stdout/stderr attached instead of surfacing as an opaque timeout.
- Bun's fs.rm ignores maxRetries/retryDelay, so fixture removal retries
  EBUSY/EPERM/ENOTEMPTY itself within a bounded budget and throws with context.
- Add the `memory-two-process` Windows flake-soak target.

Part of #8294