chore(sync): migrated kernel atomics to the typed atomic wrappers

[FlareCoding]

Note

Medium Risk
Mechanical but very broad changes in scheduler, signal delivery, and SMP boot synchronization; any mismatched memory order or non-atomic access on a formerly shared field could cause rare races or AP boot hangs.

Overview
Replaces widespread __atomic_* and volatile flag usage with sync::atomic<T> and sync::atomic_ref across the kernel (scheduling, signals, SMP, timers, IRQ/MSI dispatch, networking, PTY, reaper, sync primitives, and tests).

Type-level changes: hot shared fields are now atomics in structs—e.g. sched::task state, cleanup_stage, run_ticks; signal blocked/pending and group shared_pending/exit_signal; smp::cpu_info::state; poll/mutex owner, triggered, error; PTY/console foreground group; MSI handler slots; DHCP RX ready/active.

Access pattern: standalone counters and flags use sync::atomic methods (load_acquire, store_release, fetch_add_relaxed, cmpxchg_strong_acq_rel, etc.). Fields that stay plain scalars for layout/assembly (e.g. task_exec_core::on_cpu, exec.cpu, per-CPU epochs) are updated via sync::atomic_ref<uint32_t> / uint64_t.

Fences: IRQ registration and similar handoffs use sync::atomic_fence_release/acquire; kill/block and signal wake paths use a new sync::atomic_fence_seq_cst. A few failed-CMpxchg paths add an explicit acquire fence when adopting another thread’s winning value (e.g. group exit signal / exit_group status).

Test helpers and kernel tests switch from volatile + builtins to the same typed atomics for handshakes.

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