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/expected/1805-concurrency-io-blocking-async.exit

@@ -0,0 +1 @@
+0

examples/expected/1805-concurrency-io-blocking-async.stderr

@@ -0,0 +1 @@
+

examples/expected/1805-concurrency-io-blocking-async.stdout

@@ -0,0 +1,3 @@
+sum: 42
+double: 42
+clock ok

examples/expected/1806-concurrency-io-cancel.exit

@@ -0,0 +1 @@
+0

examples/expected/1806-concurrency-io-cancel.stderr

@@ -0,0 +1 @@
+

examples/expected/1806-concurrency-io-cancel.stdout

@@ -0,0 +1,2 @@
+ok: 7
+canceled: -99