Animation Layers - Support for simultaneous skeletal animations with blending #833

Problem this feature should fix

Currently, CSkinnedMeshRenderer can only play one animation at a time. This limitation prevents developers from:

1. Layering independent animations: Cannot play multiple animation tracks simultaneously (e.g., base locomotion + upper body action, or idle + gesture)
2. Reusing animation clips across contexts: Animation combinations must be pre-baked as separate clips instead of dynamically composed
3. Smooth animation blending: No built-in support for weighted blending between multiple poses

This results in animation clip duplication, increased asset management complexity, and reduced runtime flexibility.

Expected solution

Implement an Animation Layers system in CSkinnedMeshRenderer:

Features:

• Multiple simultaneous animation layers per renderer
• Each layer operates independently with its own playback state:
  • animationIndex / animationName
  • currentTimeTicks (per-layer animation time)
  • playbackSpeed (per-layer speed control)
  • isPlaying / isLooping (per-layer playback mode)
  • weight (0.0–1.0, contribution to final pose blending)
• Blend skeletal poses at the bone level using:
  • Linear interpolation (Lerp) for position and scale channels
  • Spherical linear interpolation (Slerp) for rotation channels
• Automatic weight normalization (sum to 1.0)