[Fix] threading_posix: reap thread handle once (mission-end freeze)

post_execution() runs on every successful Wait() and a Thread handle stays
signaled forever after exit, so multiple guest waits on the same exited handle
each call pthread_join() on an already-reaped pthread_t. On glibc that hangs
(recycled tid) -> the black-screen freeze at mission teardown. Guard reap with
an atomic exchange so it happens exactly once. Isolated from the audio/crash
WIP on phase-a-args-fileread; only this hunk, on stock 16e1eb8e2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/xenia/base/threading_posix.cc

@@ -14,6 +14,7 @@
 #include "xenia/base/platform.h"
 #include "xenia/base/threading_timer_queue.h"
 
+#include <atomic>
 #include <pthread.h>
 #include <sched.h>
 #include <semaphore.h>
@@ -981,11 +982,22 @@ class PosixCondition<Thread> final : public PosixConditionBase {
   static void* ThreadStartRoutine(void* parameter);
   bool signaled() const override { return signaled_; }
   void post_execution() override {
+    // Reap exactly once. post_execution() runs on EVERY successful Wait(), and
+    // a thread object stays signaled forever once it has exited -- so every
+    // later wait on the same handle would pthread_join() a pthread_t that the
+    // first join already reaped and freed. On glibc that faults or, if the tid
+    // was recycled, blocks forever inside pthread_join(). The guest does
+    // exactly this at mission teardown (several waiters on one thread handle),
+    // which hung the waiting guest thread: the game froze to a black screen.
+    if (reaped_.exchange(true)) {
+      return;
+    }
     if (thread_) {
       pthread_join(thread_, nullptr);
     }
     sem_destroy(&suspend_sem_);
   }
+  std::atomic<bool> reaped_{false};
   pthread_t thread_;
   pid_t tid_ = 0;                     // Kernel TID for setpriority() fallback
   mutable bool fifo_failed_ = false;  // True after SCHED_FIFO was rejected