feat: Add metrics module (PSNR, SSIM, MSE + denoiser comparison)

[KrishanYadav333]

Summary

Evaluation metrics for the EXXA denoising pipeline.

Changes

• mse(), psnr(), ssim() — standard image quality metrics
• ssim() handles (H,W) / (C,H,W) / (B,C,H,W) inputs
• compare_denoisers() — dict of {name: image} → {name: {psnr, ssim, mse}}, always includes a noisy baseline

Tests

24 tests covering zero/inf for perfect predictions, monotonic with noise level, known values, all input shapes. All pass.