Bun 1.4.2 PTY ordering

Trap: PTY lifecycle tests passing under Bun 1.3.14 looks sufficient, but its scheduling hid a race where timeout status became killed before process-group cleanup completed. Fix: run the full PTY stability gate with the pinned Bun 1.4.2 SEA builder and retain a Bun 1.3.14 compatibility run; assert semantic cleanup rather than fragile sub-80ms timer timing.