test(pty): make native wait cleanup cancellation deterministic

[code-yeongyu]

Summary

• mark native PTY sessions cancelled before cleanup signalling in the threadpool starvation fixture
• avoid a fast SIGKILL exit racing the cancellation flag observation

Root cause

The fixture sent SIGKILL through kill(signal) first. That API intentionally sends only the requested signal and does not set the cancellation flag; the follow-up no-argument kill() could lose a race with waitExit() on macOS, producing a normal exit even though the fixture was cleaning up a cancelled session. The flow-control changes altered scheduling enough to expose this existing fixture race deterministically.

Verification

• focused native wait test: 2 consecutive passes
• full @earendil-works/pi-pty suite: 9 files, 71 tests passed
• cargo test -p senpi-pty: 13 tests passed
• package TypeScript build passed
• prebuild freshness check was run and reports the existing vendored darwin-arm64 binary differs from a local rebuild; no native artifact was changed

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Summary by cubic

Fixes a race in the native PTY test fixture so cancellation is marked before the cleanup signal.

The fixture previously sent SIGKILL first, which didn't set the cancellation flag, so a fast exit could lose the race with waitExit() on macOS. It now marks cancellation with kill() before sending kill("SIGKILL"), making cleanup deterministic.

^{Written for commit f3afea9. Summary will update on new commits.}