[Feature] GPU skinning

[vmarcella]

Overview

Add GPU-based mesh skinning that deforms vertex positions based on bone
transforms for rendering animated characters.

Current State

No response

Scope

Goals:

• Vertex shader skinning
• Support up to 4 bone influences per vertex
• Skin matrix uniform buffer
• Integrate with existing pipeline system

Non-Goals:

• CPU skinning
• Morph targets/blend shapes
• Dual quaternion skinning

Proposed API

ub struct SkinnedMesh {
mesh: Mesh,
joint_indices: Vec<[u8; 4]>, // per-vertex bone indices
joint_weights: Vec<[f32; 4]>, // per-vertex bone weights
}

pub struct SkinningData {
matrices: Vec<[[f32; 4]; 4]>, // MAX_BONES matrices
}

impl SkinningData {
pub fn from_pose(pose: &Pose, skeleton: &Skeleton) -> Self;
pub fn to_buffer(&self) -> Vec;
}

// Vertex shader receives joint_indices and joint_weights as attributes
// Uniform buffer contains skin matrices

### Acceptance Criteria

- [ ] Skinned mesh deforms with bone movement
- [ ] Multiple bone influences blend correctly
- [ ] Skin matrices uploaded each frame
- [ ] Shader correctly transforms vertices
- [ ] Example rendering animated character

### Affected Crates

lambda-rs

### Notes

- MAX_BONES typically 64-256
- Joint indices packed as u8 or u16
- Weight normalization (should sum to 1.0)