Fix iOS player crash due to mimalloc symbol conflict in Unity 6.5+

[ruccho]

Problem

After updating Unity to 6000.5.2f1, iOS builds crash on launch (EXC_BAD_ACCESS → watchdog SIGKILL).

Unity 6000.5 now bundles its own mimalloc in UnityFramework (_heap_main, _mi_page_empty, etc.). Our libunienc_c.a bundles its own mimalloc too, and on iOS it's statically linked (__Internal) into the same UnityFramework. The identically-named symbols coalesce at link time, so unienc's mimalloc ends up using Unity's _heap_main (different ABI layout) and dereferences a bogus pointer. -fvisibility=hidden doesn't help — on Mach-O it only makes symbols private-external, which still coalesce.

Fix

Localize every non-FFI symbol in the iOS .a as a CI post-build step, keeping only _unienc* global. No submodule/source changes.

• apple-exports.txt — keeps _unienc* exported (all FFI entry points, incl. unienc_UnityPluginLoad/Unload).
• localize-apple-staticlib.sh — ar x → ld -r -exported_symbols_list (demotes non-_unienc* globals to local) → libtool -static. A nm -gU gate fails the build if any mimalloc symbol is still external.
• build-unienc.yml — runs the script in build-ios (Unity variant), and sets CFLAGS_aarch64-apple-ios: -fno-common.

Linker notes (ld-prime, Xcode 16+)

• Use loose objects, not -force_load (it localizes everything, wiping the exports).
• -platform_version is required even for -r.
• -fno-common is required: _mi_page_map is a common symbol otherwise and can't be localized. Injected via cc-rs CFLAGS, so no submodule change.

Verification

nm -gU shows only the 34 _unienc* symbols and 0 mimalloc symbols. Verified on device: launches on Unity 6000.6.0b1, Memory Profiler integration retained.