♻️ Refactoring

• Core: Extracted spectral matrix computation into FilmStock::compute_spectral_matrix(), eliminating duplicated spectral integration logic across pipeline and processor modules.

⚡️ Performance

• Spectral: Added SIMD (f32x4) optimization to Spectrum arithmetic operators (Add, Mul), reducing per-operation cost for spectral calculations.

✨ Features

• GPU: Enhanced develop shader with spectral/color dual matrix pipeline, shoulder softening compression, and logistic sigmoid H-D curve (replacing erf approximation) for better CPU/GPU consistency.
• Grain: Switched grain shader shadow noise model from inverse-distance to exponential decay (exp(-2D)), providing smoother shadow-to-midtone grain transitions.

🐛 Fixes

• Metrics: Fixed PSD slope calculation to use correct row-then-column 2D FFT instead of flattened 1D FFT.