fix(shaders): Resolve WGSL validation errors and add shader compilation tests

Fixed three critical WGSL shader issues causing demo64k and test_3d_render to crash:

1. **renderer_3d.wgsl**: Removed dead code using non-existent `inverse()` function
   - WGSL doesn't have `inverse()` for matrices
   - Dead code was unreachable but still validated by shader compiler
   - Also removed reference to undefined `in.normal` vertex input

2. **sdf_utils.wgsl & lighting.wgsl**: Fixed `get_normal_basic()` signature mismatch
   - Changed parameter from `obj_type: f32` to `obj_params: vec4<f32>`
   - Now correctly matches `get_dist()` function signature

3. **scene_query_linear.wgsl**: Fixed incorrect BVH binding declaration
   - Linear mode was incorrectly declaring binding 2 (BVH buffer)
   - Replaced BVH traversal with simple linear object loop
   - Root cause: Both BVH and Linear shaders were identical (copy-paste error)

Added comprehensive shader compilation test (test_shader_compilation.cc):
- Tests all production shaders compile successfully through WebGPU
- Validates both BVH and Linear composition modes
- Catches WGSL syntax errors, binding mismatches, and type errors
- Would have caught all three bugs fixed in this commit

Why tests didn't catch this:
- Existing test_shader_assets only checked for keywords, not compilation
- No test actually created WebGPU shader modules from composed code
- New test fills this gap with real GPU validation

Results:
- demo64k runs without WebGPU errors
- test_3d_render no longer crashes
- All 22/23 tests pass (FftTest unrelated issue from FFT Phase 1)

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

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