Cross-platform C++ SDK with a C-ABI boundary and XCFramework pipeline

Why this shape

A native SDK meant to outlive one client app has one real design constraint: the public surface must be pure C. C++ has no stable ABI across compilers/versions, so any header that leaks std::string, templates, exceptions, or virtual dispatch across the library boundary locks every consumer to the exact toolchain that built the library. A pure C header (extern "C", POD structs, opaque handles, function-pointer callbacks) is readable from Swift, Kotlin/JNI, C#, or plain C — so the same compiled core serves an iOS/macOS app today and an Android .aar or Windows .dll later without a rewrite. Everything in this skill exists in service of that one constraint; the patterns below (Pimpl, interfaces, factory, callbacks) are how you get to write normal, comfortable C++ underneath that boundary without it leaking through.