feat(shaders): Add bump-mapped normal calculation variants

Adds two new functions for displacement-mapped normal calculation:

Functions added:
- get_normal_bump(): High-quality 6-sample central differences
- get_normal_bump_fast(): Optimized 4-sample tetrahedron pattern

Performance comparison:
- get_normal_bump: 6 SDF + 6 texture + 6 UV = 18 operations
- get_normal_bump_fast: 4 SDF + 4 texture + 4 UV = 12 operations
- Speed improvement: 33% faster with tetrahedron method

Parameters:
- p: Sample position (vec3)
- obj_params: Object parameters (vec4)
- noise_tex: Displacement texture (texture_2d)
- noise_sampler: Texture sampler
- disp_strength: Displacement strength multiplier

Requirements:
- spherical_uv() function must be available in calling context
- get_dist() function must be available in calling context

Use cases:
- get_normal_bump: Static scenes, high-quality renders
- get_normal_bump_fast: Real-time rendering, performance-critical paths

Location: assets/final/shaders/math/sdf_utils.wgsl

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