[codex] add async tasklet waker

[xuchang-vivo]

Summary

• add a per-tasklet RawWaker for the kernel async executor
• requeue pending tasklets only when their waker fires
• add a yield_now future and unit coverage

Tasklet State Model

The async executor tracks two things for each tasklet: whether it is currently linked in ASYNC_WORK_QUEUE, and the tasklet scheduling state.

stateDiagram-v2
    [*] --> IDLE: created

    IDLE --> QUEUED: wake and enqueue
    QUEUED --> POLLING: pop from queue and poll

    POLLING --> IDLE: pending and not woken
    POLLING --> WOKEN: woken during poll
    WOKEN --> QUEUED: requeue after pending

    POLLING --> COMPLETED: ready
    COMPLETED --> [*]: no Arc refs remain

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Queue ownership by state:

State	In async work queue?	Meaning
IDLE	No	The tasklet is pending but not scheduled. A later waker call can enqueue it.
QUEUED	Yes	The tasklet is already scheduled. More wake calls are coalesced.
POLLING	No	The poller popped it from the queue and is currently polling its future.
WOKEN	No	The tasklet was woken while it was being polled; it will be requeued after Pending.
COMPLETED	No	The future returned Ready and will not be scheduled again.

pop_front() does not destroy the tasklet. It transfers the list-owned Arc<Tasklet> into the local task variable in poll_inner(). If the future stores cx.waker(), that waker also owns an Arc<Tasklet>, so future events can schedule the same tasklet again.

Validation

• ninja -C out/qemu_riscv64.release kernel/kernel:check_kernel_by_clippy

[xuchang-vivo]

build_prs

[github-actions]

Job is started, see https://github.com/vivoblueos/kernel/actions/runs/25146859694.

[github-actions]

✅ All jobs completed successfully, see https://github.com/vivoblueos/kernel/actions/runs/25146859694.

[han-jiang277]

pls add more test cases

[han-jiang277]

pls add async design in book.