Close the pty write-after-respawn race and bound the pty PATH resolve stall

[rakibulism]

Summary

• ptySpawn now returns a versioned PtyHandle{key, generation}; a new ptyWriteChecked refuses a write whose generation no longer matches the current occupant of that key. This closes a misdelivery hazard where a stale write (issued for an old pty occupant) could silently land on a new pty that reused the same key. ts_core_host.zig's spawn/write path now uses it.
• resolveExecutable's PATH walk (blocking access() calls with no timeout) now runs on a bounded worker thread via resolveExecutableBounded, capped at 300ms before the worker is abandoned — a stalled NFS/autofs mount now costs the loop thread a bounded stall instead of hanging indefinitely.
• Adds a real-executor test for the deferred (non-inline) host-request answer path racing cancelHostRequest, closing a coverage gap in effects_host_tests.zig — previously every real-executor request_fn stub answered synchronously inline, so the genuinely async path (feedHostResult called later, off request_fn's own call stack) was untested.

Test plan

• zig build test passes locally (Zig 0.16.0) with the same set of pre-existing, environment-gated failures (missing codesigning identity, no Android SDK, etc.) as an unmodified checkout — nothing new broken.
• zig ast-check clean on every edited file.
• The real-transport pty test suite (which exercises resolveExecutableBounded on every spawn) runs and passes.

[vercel]

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