feat: spring bones

[daniele-dcl]

Summary

• Replace hardcoded BONE_COUNT = 62 with BASE_BONE_COUNT = 62 and MAX_BONE_COUNT = 256 to support variable bone counts per avatar
• Make BoneArray accept variable-length bone arrays within the valid range
• Implement dynamic bone stride in AvatarTransformMatrixJobWrapper — automatically resizes when an avatar with more bones joins
• Thread boneCount through the entire compute skinning pipeline: Initialize() → SetupCounters() → SetupBuffers() → CopyAllBuffers() → ComputeSkinning()
• Store per-avatar BoneCount in AvatarCustomSkinningComponent and use boneStride from the job wrapper for correct GPU buffer offsets

Context

The avatar compute skinning pipeline hardcoded BONE_COUNT = 62 (the standard Decentraland skeleton). Spring bones (via UniVRM) add extra bones to wearables. This PR makes the bone count variable per avatar so wearables with spring bones can be skinned through the same GPU pipeline.

The approach uses a max-stride shared across all avatars in the parallel job array. When a new avatar exceeds the current stride, the buffers are reallocated and all avatars skip one frame (invisible stale pose). Most avatars still have 62 bones so wasted slots are minimal.

Prerequisite (not in this PR): AttachmentAssetUtility.RemoveBonesGameObjects() currently destroys all child GameObjects without a Renderer, which would delete spring bone transforms. That must be addressed separately.

Test plan

• Project compiles with no references to deleted BONE_COUNT constant
• All existing avatar instantiation tests pass (standard 62-bone path unchanged)
• Manual test: equip a standard avatar → verify skinning renders correctly (regression)
• Future: equip wearable with spring bones once AttachmentAssetUtility fix is in place

🤖 Generated with Claude Code

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