feat: Separation and DeviceN color spaces (ISO 32000-2 §8.6.6-7) #28

[ralflang]

Summary

• Add PDF function infrastructure (PdfFunction interface, ExponentialFunction Type 2, PostScriptFunction Type 4)
• Implement SeparationColorSpace for single spot colors (Pantone etc.)
• Implement DeviceNColorSpace for multiple spot colors
• Generalize ResourceDictionary and ContentStreamBuilder from IccBasedColorSpace to ColorSpace interface
• PdfSerializer emits /ColorSpace resource dict entries with polymorphic dispatch for ICC, Separation and DeviceN

Test plan

• ExponentialFunction stores c0/c1/exponent, returns correct type/domain/range
• PostScriptFunction stores code/domain/range
• SeparationColorSpace has componentCount 1, stores colorant/alternate/transform
• DeviceNColorSpace componentCount matches colorant array
• SeparationColor emits correct cs/sc operators
• DeviceNColor validates tint count, emits correct operators
• Serializer produces [/Separation /name /alternate fnRef] array objects
• Serializer produces [/DeviceN [...] /alternate fnRef] array objects
• Type 2 function serialized with /FunctionType 2, /C0, /C1, /N
• Type 4 PostScript function stream serialized correctly
• Full regression suite passes (570 tests)