Compute hillshade illumination for a terrain grid.

Implements the standard hillshade algorithm used in GIS:

1. Compute terrain gradient (dz/dx, dz/dy)
2. Calculate illumination angle from azimuth + altitude
3. Shade = max((cos(zenith) * cos(slope) + sin(zenith) * sin(slope) * cos(azimuth - aspect)), 0)

Arguments

• heights: Float32Array elevation grid (row-major)
• width: Grid width (columns)
• height: Grid height (rows)
• azimuth_deg: Light azimuth in degrees (0 = North, 90 = East)
• altitude_deg: Light altitude/elevation in degrees (90 = directly above)

Returns

Uint8Array of illumination values (0 = shadow, 255 = full light).