perf(studio): no findings stall every few seconds

[sp00ktober]

The findings re-check parsed the built PAC's 103 clips twice on the GUI thread every ~3.7 s: a 250–480 ms stall in every tab, seen as skipped frames in clip playback and hitches while panning (since #31).

The clip table is now parsed once per PAC (read-only), and a timed re-check that costs over 50 ms waits a minute before the next; an edit still re-checks at once.

Measured with a 10 ms stall probe:

• offscreen, idle 60 s: 16 stalls (~5.7 s frozen) → 0;
• panning the Behaviour canvas 45 s: 14 stalls → 1 (the first draw);
• GL clip playback under Xvfb 60 s: 14 frame gaps over 50 ms → 0, 41 → 57 paints/s.

🤖 Generated with Claude Code