Dirt RT

feat: groundbreaking denoiser — SG-based physical irradiance integration + SVGF with Mahalanobis edge-stopping

This is a landmark release that rewrites the entire denoising pipeline from the

ground up. The new denoiser produces dramatically cleaner, more stable, and

more physically accurate results than ALL previous versions combined.

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CORE INNOVATION: Spherical Gaussian (SG) Physical Irradiance Integration

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The old SH (Spherical Harmonics) irradiance projection has been completely

replaced with a physically-based SG (Spherical Gaussian) integration framework:

• Dual-SG product convolution — light lobe × cosine lobe solved analytically

via SGProduct(), yielding exact attenuation without series truncation

• HSG (Hemispherical Spherical Gaussian) integral — closed-form integration

over the hemisphere using a rational approximation of the erf function

• NaN-safe math primitives throughout — precise_erf() with Abramowitz-Stegun

coefficients, expm1_over_x() with 1st-order Taylor fallback, and