feat: Detect incorrect multi-threaded use from c based ffi (#499)

Add runtime detection for shared handle misuse

wrap the FFI engine handle with parking_lot::RwLock when the new
contention_checks feature is enabled, surfacing a clear “handle is already
in use” error instead of allowing undefined behavior
keep the feature optional so no_std builds or environments that supply
their own synchronization can opt out
caution users that this guards the handle itself but does not make the
engine’s operations globally thread-safe on its own

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

bindings/ffi/src/allocator.rs

@@ -9,7 +9,7 @@ extern "C" {
 
 #[cfg(feature = "custom_allocator")]
 mod allocator {
-    use std::alloc::{GlobalAlloc, Layout};
+    use core::alloc::{GlobalAlloc, Layout};
 
     struct RegorusAllocator {}