Add platform/src/cg.rs with: - CGColorSpace (device RGB), CGBitmapContext, CGImage wrappers - CGRect/CGPoint/CGSize geometry types - BitmapContext: Rust-owned pixel buffer wrapped in CGBitmapContext - fill_rect(), clear(), create_image(), pixels/pixels_mut accessors - draw_image_in_context() for blitting into a view's CG context - 8 unit tests covering context creation, drawing, and edge cases Add custom NSView subclass (WeView) in appkit.rs: - Overrides drawRect: to blit CGImage from BitmapContext - Overrides isFlipped for top-left coordinate origin - BitmapView wrapper with set_needs_display() for redraw requests - Uses ivar to store BitmapContext pointer per-instance - Window::set_content_view() for installing custom views - 2 new tests for WeView class registration and BitmapView creation Add ivar support to objc.rs: - class_addIvar FFI binding - object_setInstanceVariable / object_getInstanceVariable FFI - Id::set_ivar() and Id::get_ivar() helper methods - Class::add_ivar() helper method Update browser main.rs: - Creates 800x600 BitmapContext - Fills dark gray background with centered blue rectangle - Installs BitmapView as window content view Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>