Function schedule_select

* Select next thread to run with PTS enforcement. * * O(1) selection: CLZ gives highest priority, return queue head. * Strict invariant: all queued threads are runnable. * * PTS Enforcement: Task j preempts task i iff π_j < γ_i * - If current thread has preemption threshold set, only threads with * priority < threshold can preempt * - Deferred threads are tracked in preempted_bitma