fibers B1.2 COMPLETE: async/await/cancel examples (1805/1806)

With the three surface blockers fixed (0151 generic inference, 0152
Atomic(bool), 0153 re-export failable channel), the M:1 async surface works
end-to-end on the blocking Io default. Landed the corpus examples:

- 1805-concurrency-io-blocking-async.sx: context.io.async(lambda, ..args)
  runs the worker inline, await() or {…} yields the result; context.io.now_ms()
  reads the monotonic clock. Prints sum: 42 / double: 42 / clock ok.
- 1806-concurrency-io-cancel.sx: f.cancel() marks the future canceled so a
  later await() raises error.Canceled out of its (R, !IoErr) channel, caught
  with or. Prints ok: 7 / canceled: -99.

B1.2 (Io capability on Context + async/await/cancel + blocking CBlockingIo) is
complete. Suite green 732/0. Next: B1.3 (fiber runtime).

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

@@ -0,0 +1,23 @@
+// B1.2 — the async ergonomic layer over the `Io` capability, blocking
+// default. `context.io.async(worker, ..args)` runs the worker to completion
+// inline and returns a `.ready` Future($R); `f.await()` yields the result
+// (a value-failable `($R, !IoErr)`, handled with `or`). `context.io.now_ms()`
+// reads the monotonic clock through the same capability.
+//
+// Worker form: a lambda whose params are annotated at the call site
+// (`(a: i64, b: i64) -> i64 => …`); `..args` forwards the call-site
+// arguments to it.
+#import "modules/std.sx";
+
+main :: () {
+    // Homogeneous args.
+    s := context.io.async((a: i64, b: i64) -> i64 => a + b, 40, 2);
+    print("sum: {}\n", s.await() or { -1 });
+
+    // Single arg.
+    d := context.io.async((x: i64) -> i64 => x * 2, 21);
+    print("double: {}\n", d.await() or { -1 });
+
+    // The Io capability also carries a monotonic clock.
+    if context.io.now_ms() >= 0 { print("clock ok\n"); }
+}