[WIP] Fix MIS implementation for environment map sampling

[Copilot]

• Analyze codebase and understand current environment map sampling implementation
• Fix binary search precision - increase iterations from 8 to 16
• Fix PDF calculation - add sin(θ) Jacobian factor for spherical coordinate correction
• Implement environment map importance sampling function
• Enable commented out importance sampling code with proper PDF correction
• Implement Multiple Importance Sampling (MIS) with power heuristic (β=2)
• Add MIS weighting between BSDF and environment map sampling strategies
• Test changes and verify improved convergence

Current Issues Found:

1. Environment map importance sampling is commented out (line 398)
2. PDF correction is disabled (lines 403-404)
3. Binary search uses only 8 iterations (insufficient precision ~1/256)
4. PDF calculation lacks sin(θ) Jacobian factor for spherical coordinates
5. No MIS weighting between BSDF and light sampling strategies

Implementation Plan:
The changes will be focused on the raytrace.wgsl shader file. Key functions to modify:

• getEnvironmentMapUV() - increase binary search precision to 16 iterations
• getEnvironmentMapPDF() - add sin(θ) Jacobian correction
• trace() - implement MIS between BSDF and environment map sampling
• Add new sampleEnvironmentMap() function for importance sampling

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