fix(ik): cache camera exposure decisions and reject hidden picks (#68)

[HaD0Yun]

Closes #68.

What was measured

Real Chrome (ANGLE Metal, Apple M4 Pro) over CDP with the new harness tools/perf/ik-camera-drag.mjs: character with motion loaded, IK ON, a 2 s synthetic right-button drag, frame times sampled via rAF. Three runs each on the unfixed tree and on this branch, at DPR 1 and at DPR 2 (1920x1200 viewport).

tree	median	p95	fps
main 8f0c5c7 (DPR 2)	16.7 ms	16.8 ms	56.7-57.2
this branch (DPR 2)	16.7 ms	16.8 ms	56.2-56.7

The camera-drag path is already vsync-bound on current main: the substantive optimisations for this issue landed after it was filed (6daea2d, 1401e0e, 4be7cd7, 56baded — exposure skipped during camera gestures, deferred outline passes, reduced DPR while navigating, frozen shadow maps). A CPU profile during the drag shows only (idle)/(program)/React JSX construction. So the honest relative improvement is 0%; the absolute result is above 55 fps.

What this branch changes

• shouldRefreshIkExposure() (src/use-render-activity.js) makes the exposure refresh policy explicit and unit-testable; posestudio.jsx uses it in the existing hot path so a future change cannot silently reintroduce per-frame proxy scans during camera motion.
• Pointer picking now excludes handles whose userData.ikExposed === false: an occluded handle could still capture focus (verify-ik-browser case "hidden shoulder cannot take its own focus" now passes).
• test/verify-ik-camera-performance.mjs (node, in npm test) and the perf harness + recorded JSON artifacts under .omo/qa/.

Remaining verify-ik-browser failures (hand manipulator setup) are pre-existing on main and unrelated.