Phase 0 of the fluxa-android KMP/CMP migration needs an xcframework +
Swift UniFFI bindings for fluxa_core. Adds an "ios" feature (full-api +
uniffi-bindings, no jni/dolby_vision, mirroring the existing "desktop"
feature), a staticlib crate-type for xcframework embedding, a
[bindings.swift] uniffi.toml section, and a macOS-only build script
that produces FluxaCore.xcframework for aarch64-apple-ios and
aarch64-apple-ios-sim.
Verified on Linux: `cargo check --target aarch64-apple-ios --features
ios` and the aarch64-apple-ios-sim equivalent both compile cleanly, so
the crate itself has no Android/JNI-only code leaking into the iOS
feature set. Actual linking/xcframework packaging needs a Mac with
Xcode — untested here.
