feat: separable Gaussian blur (arbitrary radius)

[0717lee]

Adds Image::gaussian(radius):

• Binomial C(2r,k) weights = discrete Gaussian, integer-exact normalization by 4^r
• Horizontal + vertical 1D passes: O(r)/pixel vs O(r^2) for 2D convolve
• Radius clamped to [0,32]; radius 0 = copy; alpha preserved
• 4 new tests: exact identity, flat-field fixed point across radii, impulse symmetry + compact support, radius-1 parity with Kernel::gaussian_blur within 1 rounding step
• 85/85 tests passing

Closes #21