Add pluggable color picker renderers

[vpapenko]

Summary

• add a complete pluggable IColorPickerRenderer architecture with semantic drawing contexts
• move the existing appearance into the inheritable ClassicColorPickerRenderer
• support XAML-bindable high-level renderer settings and selective Draw* overrides
• forward renderers through composite controls with binding/lifetime-safe invalidation
• isolate every renderer callback with an SKPictureRecorder
• replace slider paint factories with renderer-owned semantic gradients
• add sample renderers, UI regression coverage, and customization documentation

Validation

• 1,369 Core unit tests passed
• 14 targeted renderer and pixel-diff UI tests passed
• Windows sample and Android library builds passed
• untouched main passed all six golden rendering scenarios against the generated renderer outputs
• full local UI run: 212 passed; the remaining 10 are the existing 125%-DPI debug-outline corner-sampling artifact