Implement priority bitmap scheduler

[jserv]

This replaces slot-based scheduler with priority bitmap design for O(1) highest-priority thread selection.

• 32-level priority with per-level circular doubly-linked ready queues
• CLZ-based bitmap scanning (branch-free, handles empty bitmap)
• Strict queue invariant: only T_RUNNABLE threads in ready queues
• IRQ-safe critical sections with irq_save_flags/irq_restore_flags
• Conditional bitmap updates (only when queue transitions empty/non-empty)
• Round-robin rotation via sched_yield() for same-priority fairness
• Atomic priority changes via sched_set_priority() with queue migration

Scheduler correctness based on formal verification from:
"Formal Analysis of FreeRTOS Scheduler on ARM Cortex-M4 Cores"
Chen-Kai Lin and Bow-Yaw Wang, Academia Sinica