Map Colour in the C# plugin SDK type bridge

The C# Plugin SDK's value bridge maps a deliberately small signature set (Boolean, Integer, Double, String, MemoryBlock, Picture, arrays, exported enums and wrapper classes). Objo's native Colour type is not on that list, so a plugin author cannot declare an exported member that takes or returns a colour value directly. The member is rejected at generate time with a diagnostic naming the supported set, never silently, but authors must convert at the boundary themselves (RGB integer components, or #RRGGBB/#RRGGBBAA hex strings).

Colour is a common parameter for plugin APIs (charts, reports, document models, drawing effects). Map it so an exported C# member can use a colour value and Objo code passes and receives the native Colour type on both sides of the call.