Add last-error reporting to FFI (iOS + Android)

[AYastrebov]

Problem

shoes_start returns immediately but errors happen asynchronously (config parsing failures, tunnel setup errors, runtime crashes). There was no way for the host app to retrieve the error message after the fact.

Solution

Add a LAST_ERROR global (OnceLock<parking_lot::Mutex<Option<String>>>) in ffi/common.rs shared by both platforms.

Lifecycle: clear on start → set on async failure → read by app.

iOS

• shoes_get_last_error() returns a heap-allocated C string via CString::into_raw(), or NULL if no error
• Caller must free with shoes_free_string(ptr) — the Swift wrapper always pairs these calls
• shoes_free_string() added for safe deallocation

Android

• ShoesNative.getLastError() returns String? via JNI
• Returns JString::null() if no error stored

Files changed

File	Change
src/ffi/common.rs	LAST_ERROR global + set/clear/get_last_error() + tests
src/ffi/ios.rs	shoes_get_last_error() + shoes_free_string() exports
src/ffi/android.rs	ShoesNative.getLastError() JNI implementation
src/ffi/mod.rs	Include common module in test builds
include/shoes.h	C header with shoes_get_last_error + shoes_free_string
ShoesNative.kt	Kotlin getLastError() external declaration

Test plan

• cargo test --lib --features ffi ffi::common — 3 unit tests pass (set/get, clear, overwrite)
• iOS: call shoes_get_last_error() after a failed start, verify error string, free with shoes_free_string()
• Android: call ShoesNative.getLastError() after a failed start, verify non-null error string

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