perf: eliminate GC pressure in touch/pointer handlers

[Avatarsia]

Summary

• Pre-allocate 7 Three.js temp objects (Plane, Ray x2, Vector3 x2, Vector2 x2) to replace per-event new calls in touch/pointer handlers
• Mutate _lastPointer and _pinchMid objects instead of creating new { x, y } literals on every event
• Reuse scene.background.set() instead of new THREE.Color() in theme switching
• Reuse _tmpSize / _tmpV5 in fitCamera and setProjection

Impact: Eliminates ~360 heap allocations/second during touch drag (pinch-zoom, pan, orbit) on mobile devices. Zero allocations in hot-path event handlers after this change.

Test plan

• node --check js/viewer.js passes
• Playwright: app loads without JS errors
• Playwright: orbit interaction works (pointerdown + pointermove + pointerup)
• Playwright: theme toggle works without Color errors
• No console errors across all tests

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