arena: right-click drag to orbit camera around player (3P)
Implements camera orbit by tracking a separate orbitAngle that modifies
the XZ camera offset without changing cam.rotY. This keeps the player
body rotation fixed while the camera swivels around them.
Logic:
- Accumulate orbitAngle from right-click drag (delta.x * 0.4)
- In sim, convert current XZ offset to polar, add orbitAngle, convert back
- Decay orbitAngle back to 0 when right-click releases
- Only applies in 3P mode (zoomLevel > 0)
- Don't penLock on button 2
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>