feat(shaders): Extract common WGSL utilities for better composability

Created math/common_utils.wgsl with reusable shader functions:
- transform_normal() - Normal matrix transform (2 call sites)
- spherical_uv() - Spherical UV mapping (7 call sites)
- spherical_uv_from_dir() - For direction vectors (1 call site)
- grid_pattern() - Procedural checkerboard (2 call sites)
- Constants: PI, TAU

Refactored shaders:
- renderer_3d.wgsl: 7 spherical_uv + 1 normal + 2 grid (~12 lines removed)
- mesh_render.wgsl: 1 normal transform (~3 lines removed)
- skybox.wgsl: 1 spherical UV (~2 lines removed)

Impact: ~200 bytes saved, 12 call sites deduplicated
Tests: 31/31 passing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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