chore: remove unused mutex from the deque request scheduler (#397)

The deque-backed global request scheduler (FIFO) is a Chase-Lev
work-stealing deque used as single-producer / multi-consumer: only the
listener thread pushes (the owner end) and worker threads only steal (the
lock-free opposite end, coordinated by a CAS). deque_pop -- the other
owner operation -- is never used, so the multi-owner case the deque
comments warn about does not arise, and no lock is required.

global_request_scheduler_deque_mutex (carrying a "Should this be used???"
TODO) was never locked anywhere -- dead code. Remove it and document why
the structure is safe without a lock.

No functional change. Verified: builds; FIFO serves 20,000/20,000
requests as 200s with no errors.

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

Author: bushidocodes

runtime/src/global_request_scheduler_deque.c

@@ -6,12 +6,15 @@
 
 static struct deque_sandbox *global_request_scheduler_deque;
 
-/* TODO: Should this be used???  */
-static pthread_mutex_t global_request_scheduler_deque_mutex = PTHREAD_MUTEX_INITIALIZER;
-
 /**
- * Pushes a sandbox to the global deque
- * @param sandbox_raw
+ * Pushes a sandbox to the global deque.
+ *
+ * No lock is required: this is a work-stealing deque used as a single-producer / multi-consumer queue. Only the
+ * listener thread ever pushes (the "owner" end), while worker threads only ever steal (the lock-free opposite
+ * end, coordinated by a CAS). The owner's other operation, deque_pop, is never used, so the multi-owner case the
+ * deque comments warn about does not arise here.
+ *
+ * @param sandbox
  * @returns pointer to sandbox if added. NULL otherwise
  */
 static struct sandbox *