ProPhotoRGB builtin transforms

[ssh4net]

Add ProPhotoRGB (ROMM RGB) as a set of builtin transforms for color space conversions between ProPhotoRGB and ACES2065-1, including both native gamma 1.8 and sRGB gamma variants. Updates documentation: usage and available styles, registers the transforms in the registry. Adds comprehensive unit tests for registration, gamma curves, round-trip, and variant support.