Tier 2: robust mutex cleanup in thread exit path

exit_current_thread() now calls mark_robust_mutexes_dead(this) before
thread teardown, ensuring robust mutexes held by exiting threads are
properly marked as dead and ownership transferred to waiters.

src/pthread/mod.rs

@@ -317,6 +317,9 @@ pub unsafe fn exit_current_thread(retval: Retval) -> ! {
     unsafe { header::tls::run_all_destructors() };
 
     let this = current_thread().expect("failed to obtain current thread when exiting");
+
+    crate::sync::pthread_mutex::mark_robust_mutexes_dead(this);
+
     let stack_base = this.stack_base;
     let stack_size = this.stack_size;