feat: async/await colorblind over the fiber Io (Phase 2 of Io unification)

lab/sx

`context.io.async(worker)` / `await` now run over the `Io` PROTOCOL, so the
same code interleaves under the fiber scheduler or runs inline under the
blocking `CBlockingIo` — one async stack, reached purely through `context.io`.

- Protocol: `suspend_raw(park: *ParkToken)` (was by-value). A suspending impl
  records the parked execution context into `park.handle` before parking, so a
  cross-context `ready(park)` knows whom to resume; `Scheduler.suspend_raw`
  writes `self.current`, `CBlockingIo` ignores it.
- io.sx async layer rewritten colorblind: `async` submits the worker through
  `io.spawn_raw` (inline under blocking, a fiber under the scheduler) and returns
  a HEAP `*Future($R)` the worker fills later; `await` suspends via `suspend_raw`
  until ready, then returns/raises. The generic worker is bridged to spawn_raw's
  raw `(*void)->void` entry via a monomorphic `ThunkBox` (a heap-boxed nullary
  completion closure) — all genericity lives in the closure env. Workers are
  nullary (inputs captured at the call site) because a variadic pack can't cross
  the fiber boundary. `CBlockingIo.spawn_raw` now runs the worker inline.
- Migrated 1805/1806 to the nullary `*Future` form; retrofit 1822/1823 to the
  `push .{ … }` partial-context literal (inherits allocator/data).
- The async machinery adds a few prelude types, shifting the type-name table —
  40 `.ir` snapshots regenerated (no behavior change; only `.exit`/`.stdout`/
  `.stderr` would signal that, and none changed).

Locked by examples/concurrency/1824 — two async tasks under the fiber Io, the
completion log proving deferral (1 2 then 10 20 then 123). Suite 829/0,
byte-identical aarch64-macOS host + aarch64-linux container.

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