fibers B1.2: Io capability + context.io + blocking impl + Future/async/await/cancel

Threads an `Io` capability onto `Context` exactly like `Allocator`: a
`protocol #inline` whose process-wide default is a stateless `CBlockingIo`
(the mirror of `CAllocator`), installed in `__sx_default_context`.

Library (library/modules/std):
- core.sx: `Io` protocol (spawn_raw / suspend_raw / ready / poll / now_ms /
  arm_timer) + `SpawnOpts` / `PinTarget` / `ParkToken`; `Context` gains an
  `io: Io` field LAST (allocator stays index 0, data stays index 1).
- io.sx (new): `CBlockingIo` + `impl Io` (blocking M:1 semantics — now_ms is
  a real monotonic clock, the rest are no-ops/0; suspend never called);
  `Future($R)` { value; state: FutureState; err: IoErr; park; task; canceled:
  Atomic(bool) } with `Value :: R`; the async ergonomic layer
  `async` / `async_void` / `await` (value-carrying `(R, !IoErr)`) / `cancel`.
  Built with the verified `= ---` + field-assign + `Closure(..$args) -> $R` +
  `..$args` idiom (NON-void $R only — Future(void) is deferred per issue 0150).
- std.sx: re-export the Io surface + the io.sx tail.

Compiler (src/ir):
- protocol.zig `emitDefaultContextGlobal` + comptime_vm.zig
  `materializeDefaultContext`: both materializers of `__sx_default_context`
  now build the inline CBlockingIo->Io vtable (7 words) at the new field.
- stmt.zig `lowerPush`: `push Context.{...}` now INHERITS omitted fields from
  the ambient context (seed the slot from current_ctx_ref, overwrite only the
  literal's named fields) — correct capability-bag semantics, so the partial
  `push Context.{ allocator = X }` sites don't zero a null `io` vtable.
- protocols.zig + lower.zig + error_analysis.zig: record protocol-impl method
  names so the "declared `!` but never errors" lint skips a conforming impl
  whose `!` is dictated by the protocol contract (e.g. Io.suspend_raw).

37 `.ir` snapshots regenerated: layout-only (the Context type now carries the
Io field, shifting type-table numbering); no stdout/stderr/exit changes.

The blocking Io + now_ms + Future/async work when `async` is called with the
receiver passed explicitly; the user-facing UFCS form `context.io.async(...)`
is blocked on a separate UFCS generic-inference bug (filed next).

Suite: 726 ran, 0 failed.

library/modules/std.sx

@@ -17,6 +17,10 @@ list :: #import "modules/std/list.sx";
 
 Context         :: core.Context;
 Allocator       :: core.Allocator;
+Io              :: core.Io;
+SpawnOpts       :: core.SpawnOpts;
+PinTarget       :: core.PinTarget;
+ParkToken       :: core.ParkToken;
 Into            :: core.Into;
 Source_Location :: core.Source_Location;
 
@@ -85,6 +89,20 @@ decompose_u16x4 :: fmt.decompose_u16x4;
 
 List :: list.List;
 
+// --- Async / Io capability (impls in std/io.sx) ---
+
+io_mod      :: #import "modules/std/io.sx";
+CBlockingIo :: io_mod.CBlockingIo;
+Future      :: io_mod.Future;
+FutureState :: io_mod.FutureState;
+IoErr       :: io_mod.IoErr;
+async       :: io_mod.async;
+async_void  :: io_mod.async_void;
+await       :: io_mod.await;
+cancel      :: io_mod.cancel;
+// `timeout` / `Future(void)` are DEFERRED (B1.4) pending issue 0150
+// (a `void` struct field SIGTRAPs the compiler). Re-export once it lands.
+
 // --- The stdlib namespace tail: flat-importing std.sx carries these ---
 
 mem     :: #import "modules/std/mem.sx";