Add platform/src/cf.rs with: - Raw extern "C" bindings to CoreFoundation.framework: CFStringCreateWithBytes, CFRetain, CFRelease, CFStringGetLength, CFStringGetCString - CFStringRef type alias and CFStringEncoding constants - CfString RAII wrapper: creates CFStringRef from &str, calls CFRelease on drop - Clone impl that calls CFRetain for proper reference counting - Debug/Display impls, len/is_empty helpers, to_string_lossy for round-tripping - cf_retain/cf_release public wrappers for manual reference counting - 10 unit tests covering creation, length, empty strings, unicode, emoji, clone/drop, debug/display formatting, and pointer validity This is needed for passing string parameters to AppKit APIs (window titles, menu items, etc). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>