thread_pool: skip spurious wakeups in hill-climb monitor

pthread_cond_timedwait can return early due to spurious wakeups.
When this happens the throughput delta covers a shorter-than-expected
interval, producing an artificially low sample.  Two consecutive
spurious wakes would cause the EMA to decline twice, crossing the
decline_threshold and triggering a false direction reversal — the
monitor would shrink the pool when throughput was actually stable.

Record the wall-clock time before sleeping and discard any sample
where the elapsed time is less than half the target interval.  The
_stop check runs first so shutdown is never delayed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendored/libfuse/include/thread_pool.hpp

@@ -566,6 +566,7 @@ ThreadPool::monitor_routine(void *arg_)
     {
       // Sleep for one sample interval, but wake immediately if the
       // destructor signals _monitor_cond.
+      std::uint64_t t0 = _now_usecs();
       {
         struct timespec abs_ts;
         clock_gettime(CLOCK_MONOTONIC,&abs_ts);
@@ -587,6 +588,14 @@ ThreadPool::monitor_routine(void *arg_)
       if(btp->_stop.load(std::memory_order_acquire))
         break;
 
+      // Skip spurious wakeups.  A short interval yields an
+      // artificially low throughput delta that would cause the EMA
+      // to decline and potentially trigger a false direction
+      // reversal in the hill-climber.
+      std::uint64_t elapsed = _now_usecs() - t0;
+      if(elapsed < (std::uint64_t)interval / 2)
+        continue;
+
       std::uint64_t completed = btp->_tasks_completed.load(std::memory_order_relaxed);
       std::uint64_t raw_throughput = completed - prev_completed;
       prev_completed = completed;