refactor: retire bespoke Task async; one stack behind context.io (Phase 5)

Converge the Io unification (PLAN-IO-UNIFY Phase 5). The bespoke fiber-task layer
in sched.sx — Task / TaskState / TaskErr / go / wait / cancel(Task), plus
Scheduler.task_allocs and its deinit bookkeeping (~130 lines) — is removed. There
is now ONE async stack: context.io.async / await / cancel / race / sleep over the
Io protocol, with the Scheduler as the fiber Io's engine + driver (spawn /
yield_now / suspend_self / wake / run / block_on_fd remain as the raw primitives;
race stays in sched.sx because it needs meta.sx's make_enum/make_variant).

Migrated the four go/wait users to context.io:
- 1813 — interleave + cancel (sequence 1 2 3 42 100 -99)
- 1817 — m1 end-to-end (completion in deadline order, sum 123)
- 1819 — double-AWAIT loud-abort via the Future one-awaiter guard
- 1820 — deinit: dropped the go/task_allocs tasks; now exercises timers/io_waiters/
  kq cleanup (freed=2, live=3 = the documented per-spawn closure-env residual)

Updated readme.md (the user-facing async section documents context.io.async /
await / race / sleep) and the stale sched.go/sched.Task comments in io.sx.

Suite 854/0; no .ir churn (Task removal touched no snapshotted IR); migrated
examples byte-identical on aarch64-macOS + aarch64-linux. PLAN-IO-UNIFY Phases 0-5
all complete — the two parallel async stacks are now one, behind context.io.

examples/concurrency/1805-concurrency-io-blocking-async.sx

@@ -5,10 +5,10 @@
 // the result (a value-failable `($R, !IoErr)`, handled with `or`).
 // `context.io.now_ms()` reads the clock through the same capability.
 //
-// Worker form: a nullary lambda capturing any inputs at the CALL SITE
-// (`() -> i64 => compute(a, b)`) — the colorblind shape that also works when the
-// worker is deferred onto a fiber (a captured variadic pack can't cross the fiber
-// boundary), mirroring `sched.go`.
+// Worker form: a nullary failable lambda capturing any inputs at the CALL SITE
+// (`() -> (i64, !) => compute(a, b)`) — the colorblind shape that also works when
+// the worker is deferred onto a fiber (a captured variadic pack can't cross the
+// fiber boundary).
 #import "modules/std.sx";
 
 main :: () {