Cascaded shadow maps (CSM)

[proggeramlug]

Priority: High | Effort: Medium

Replace the single 4K shadow map with 3-4 cascades covering near/mid/far ranges.

Current state

• Single 4096x4096 shadow map, ±30 world unit frustum
• Camera-following with texel snap, 16-tap Poisson PCF
• Dynamic-offset uniform buffer for per-node transforms

What's needed

• Split camera frustum into N slices (0-10, 10-30, 30-100 units)
• One shadow pass per cascade with tighter ortho fit
• Scene shader samples correct cascade based on fragment depth
• Smooth blend between cascades

Key files

• native/shared/src/shadows.rs
• native/shared/src/renderer.rs (shadow pass + sample_shadow WGSL)