Add rotation constraints to prevent finger hyperextension in simplified hand rigs

[Copilot]

Animations designed for full 5-finger skeletons cause unnatural backward bending when applied to simplified hand models (2-3 bones). This occurs because the source animations lack anatomical constraints.

Changes

Animation constraint system (AnimationUtility.ts):

• New apply_finger_rotation_constraints() processes finger bone quaternion tracks
• Converts to Euler angles, applies anatomical limits, converts back:
  • X-axis: -0.2 to 1.8 rad (prevents hyperextension)
  • Y-axis: ±0.5 rad (lateral movement)
  • Z-axis: ±0.3 rad (axial rotation)

Integration points:

• AnimationLoader.ts: Applied during initial GLTF load
• StepAnimationsListing.ts: Applied when animations rebuild (mirroring, warping)

// Process each finger bone track
const euler = new Euler()
euler.setFromQuaternion(quat, 'XYZ')

// Clamp to prevent hyperextension
euler.x = Math.max(euler.x, -0.2)  
euler.x = Math.min(euler.x, 1.8)
euler.y = Math.max(euler.y, -0.5)
euler.y = Math.min(euler.y, 0.5)

constrained_quat.setFromEuler(euler)

The constraint values are based on typical finger joint ROM. Post-processing approach avoids runtime overhead and works universally across skeleton configurations.

Original prompt

Research this issue and see if there are any causes for fingers bending backward. #37

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
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