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.

src/ir/comptime_vm.zig

@@ -455,6 +455,24 @@ pub const Vm = struct {
             try self.machine.writeWord(addr + ps, ps, funcRefWord(fid)); // allocator.alloc_fn @ +ptr_size
         if (self.findFuncByName(module, "__thunk_CAllocator_Allocator_dealloc_bytes")) |fid|
             try self.machine.writeWord(addr + 2 * ps, ps, funcRefWord(fid)); // allocator.dealloc_fn @ +2*ptr_size
+        // Context layout: { allocator(3 words), data(1 word), io }. The inline
+        // `Io` value starts at +4*ptr_size: { ctx, fn0..fn5 }, receiver ctx is
+        // null (CBlockingIo stateless), the 6 method func-refs follow in the
+        // protocol's declaration order. Mirrors the `emitDefaultContextGlobal`
+        // global path; absent thunks (std not imported) leave the field zeroed.
+        const io_base: Addr = addr + 4 * ps;
+        const io_methods = [_][]const u8{
+            "__thunk_CBlockingIo_Io_spawn_raw",
+            "__thunk_CBlockingIo_Io_suspend_raw",
+            "__thunk_CBlockingIo_Io_ready",
+            "__thunk_CBlockingIo_Io_poll",
+            "__thunk_CBlockingIo_Io_now_ms",
+            "__thunk_CBlockingIo_Io_arm_timer",
+        };
+        for (io_methods, 0..) |mname, i| {
+            if (self.findFuncByName(module, mname)) |fid|
+                try self.machine.writeWord(io_base + (@as(Addr, @intCast(i)) + 1) * ps, ps, funcRefWord(fid));
+        }
         return addr;
     }