scheduler

Where fibers run.

A scheduler runs short tasks one at a time and fires timers. Fibers never touch threads or event loops directly; they only call schedule and call_later. That keeps the fiber runtime the same no matter what drives it:

• SyncScheduler runs everything on the calling thread and blocks while it waits for timers or for callbacks from other threads.
• AsyncioScheduler runs on an asyncio event loop.

schedule is safe to call from any thread. call_later is only called from fibers, which always run on the scheduler's own thread.

Both also serve the test clock: track counts the work a wake-up causes.

A fiber reads its scheduler and its yield budget from its context, as Effect does, so they can be provided per effect::

eff.pipe(provide_service(MaxOpsBeforeYield, 16))

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