Pressing play froze the app: the 120-frame pre-render window could
hold every shm slot in the pool (e.g. 8 workers x 3 F32 slots = 24 <
120). Once the wall-clock playhead outran the renders, the UI's
synchronous frame wait had no credit to dispatch, and the
slot-releasing cleanup runs on that same blocked UI thread — a hard
deadlock.
The window is now capped to (workers x slots - workers), reserving one
slot per worker so interactive (seek/sync display) and audio tickets
always dispatch. preview_window_capacity is exposed through
JobDispatch; a unit test pins the reserve math.
Mike-Solar
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