`impl Io for Scheduler` folds the M:1 fiber scheduler behind the same `Io`
protocol (core.sx) the blocking `CBlockingIo` implements, so the async layer
can run colorblind over whichever impl is installed via
`push Context { io = xx scheduler }`. The six methods are thin adapters over
the existing fiber primitives:
- `spawn_raw(entry, arg, opts)` — spawn a fiber that calls the erased
`(*void)->void` worker thunk `entry(arg)` (fn-ptr round-trip through
`*void`); returns the `*Fiber` handle. The fiber inherits this context
(Phase 0), so the worker's own `context.io` is this scheduler.
- `suspend_raw(park) -> !` — park the running fiber; the `!` is the
cancellation channel a suspending impl raises on (wired in Phase 3).
- `ready(park)` — `wake` the fiber recorded in the token (guarded on
`.suspended`).
- `poll(deadline_ms)` — one step of the run loop (drain ready + fire the
earliest virtual-time timer); fd-readiness stays on `run`.
- `now_ms` — the deterministic virtual clock.
- `arm_timer(deadline_ms, park)` — arm a virtual-time timer that re-readies
the current fiber.
Locked by examples/concurrency/1823 — two workers spawned + suspended +
resumed entirely through `context.io`, deterministic deadline order
(byte-identical aarch64-macOS host + aarch64-linux container). Full suite
green (828/0).
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