Add platform/src/objc.rs with: - Raw extern "C" bindings to libobjc.dylib: objc_msgSend, objc_getClass, sel_registerName, class_addMethod, objc_allocateClassPair, objc_registerClassPair, class_getName - Sel wrapper: registered selector with interned pointer equality - Class wrapper: class lookup, dynamic class creation, method addition - Id wrapper: non-null Objective-C object pointer - msg_send! macro: type-safe(ish) message dispatch via transmuted objc_msgSend (0-4 argument variants) - sel! and class! convenience macros - 12 unit tests covering class lookup, selector registration, alloc/init, custom class creation, method addition, and message dispatch This is the foundation all other platform FFI builds on. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>