fix(smoke): _exit the C client once it has its frame

libmoq.a statically bundles moq-video (openh264/vaapi/cuda). Those worker
threads use priority-protected mutexes, and tearing them down at normal
process exit can trip glibc's __pthread_tpp_change_priority assertion and
abort with a core dump, even though the moq data path already delivered a
frame. The client only needs to confirm receipt, so _exit(0) right after,
skipping the dependency's atexit teardown. Fixes the flaky rust -> c leg.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QfH4fCnLu9WHGdAjwW8L3Y

Author: claude

test/smoke/clients/c/subscribe.c

@@ -159,8 +159,13 @@ int main(int argc, char **argv) {
 
     if (got) {
         fprintf(stderr, "received a frame from %s\n", broadcast);
-        moq_session_close((uint32_t)session);
-        return 0;
+        // The data path succeeded, which is all this smoke client verifies.
+        // libmoq statically bundles moq-video (openh264/vaapi/cuda), whose
+        // worker threads use priority-protected mutexes; tearing them down at
+        // normal exit can trip a glibc pthread priority assertion and abort.
+        // Skip that atexit teardown with _exit now that we have our result.
+        fflush(stderr);
+        _exit(0);
     }
     fprintf(stderr, "error: timed out waiting for data\n");
     return 1;