Binder: PyType.getInstance cache is a data race on a free-threaded build #160

python/multiplatform/ffi/PyType.kt memoises into a plain mutableMapOf and relies on the caller holding the GIL (docs/design/threading-and-abi.md). On 3.14t two Kotlin threads creating types in parallel race on it; the desktop suite passing on 3.14t does not exercise that case.
Completion: a desktop 3.14t stress test with N threads calling PyType.getInstance concurrently that fails (or is flagged) before the fix; then a thread-safe cache. Related: #117.