fibers B1.2: lock async/await example 1805 (RED)

Adds the blocking-Io async/await example with a seeded empty .exit marker.
The example fails today (Io protocol + context.io + async/await not yet
implemented); the next commit lands the Io interface + blocking impl +
both __sx_default_context materializers + push-inherit fix to turn it green.

Worker is a lambda with annotated params (the verified B1.2 idiom);
named-fn workers are deferred pending a :: callable-param feature.

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

@@ -0,0 +1,35 @@
+// Stream B1 (fibers) — the `Io` capability + the blocking-`Io` default
+// (step B1.2). `Io` is threaded on `Context` exactly like `Allocator`: a
+// `protocol #inline` at a fixed field, whose process-wide default is a
+// stateless `CBlockingIo` (the mirror of `CAllocator`).
+//
+// In the blocking M:1 model there is no scheduler and no suspension:
+// `async(worker, ..args)` runs the worker to COMPLETION synchronously, so
+// the returned `Future` is born `.ready` and `await` yields the stored
+// result immediately. This locks the B1.2 surface — `context.io.async(...)`
+// with a lambda worker (annotated params) + `f.await()`.
+//
+// Worker form: a lambda `(a: i64, b: i64) -> i64 => ...` whose params are
+// annotated. Named-fn workers need a `::` callable-param feature that does
+// not exist yet and are DEFERRED.
+#import "modules/std.sx";
+
+main :: () -> i32 {
+    // Single-arg lambda worker.
+    f1 := context.io.async((n: i64) -> i64 => n * 2, 21);
+    v1, e1 := f1.await();
+    if !e1 { print("double: {}\n", v1); }       // → 42
+
+    // Two-arg lambda worker — exercises the `..$args` variadic forward.
+    f2 := context.io.async((a: i64, b: i64) -> i64 => a + b, 40, 2);
+    v2, e2 := f2.await();
+    if !e2 { print("sum: {}\n", v2); }           // → 42
+
+    // `now_ms` is a protocol method (a deterministic-sim Io [B1.4] can
+    // override it); the blocking Io returns a real monotonic clock, so we
+    // only assert it is non-negative, not an exact value.
+    t := context.io.now_ms();
+    if t >= 0 { print("clock ok\n"); }
+
+    return 0;
+}