Caught the freeze by waiting for the event (frozen.py + in_flight) instead of
sleeping a guessed interval; freeze_waitobj.sh splits into boot/watch so the
wait is not capped by one Bash call. Verified hard: a frame minutes later is
byte-identical to the capture.
Healthy vs frozen, same run: 20 -> 24 wait frames, XEvent 19 -> 23,
XSemaphore 8 -> 7. The signature is per-thread -- 17 of 24 threads sit on the
exact object they were on, four previously-running threads park, and T74/T75
move off a semaphore onto an event. So the freeze is not a whole-emulator stall.
Also corrects the previous entry's test: screen_id reads 'flight' during a
freeze by design, which is why frozen.py exists. Re-testing the saved frames
says that run was genuinely healthy, but it was right by luck.
heavy_read.py added to test whether the instrument provokes the freeze: I/O is
free (371 MB in 0.1s, page cache), the cost is Python-level CPU. One data point
-- 670s clean, then frozen 54s after the inducer started -- recorded as n=1, not
as causation.
0b7fd59489