Make cancellation propagation atomic

[Clyraz]

Fixes the 4th case mentioned in #1609:

• Coroutine already cancelled when a co_await starts.

I added a new state "cancelled_ptr" which will be used after cancelling.
Now the canceller will be invoked immediately if the previous value wasn't nullptr.

[Copilot]

Pull request overview

This PR addresses a remaining cancellation race in the coroutine cancellation propagation machinery by making cancellation “sticky” and ensuring newly-registered cancellers run immediately when a coroutine has already been cancelled.

Changes:

• Update set_canceller to invoke the canceller immediately when registration fails because cancellation has already occurred / is in progress.
• Introduce a new sentinel state (cancelled_ptr) and update cancel()/revoke_canceller() to use it so cancellation remains observable for future awaits.
• Refine atomic state transitions in cancel() to avoid clobbering in-progress cancellation and to record post-cancel state.