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";

library/modules/std/core.sx

@@ -64,11 +64,61 @@ Allocator :: protocol #inline {
     dealloc_bytes :: (ptr: *void);
 }
 
+// --- Io capability protocol (impls live in std/io.sx) ---
+//
+// `Io` is threaded on `Context` exactly like `Allocator`: a `#inline`
+// protocol whose default impl (the stateless `CBlockingIo` in std/io.sx)
+// is installed in the process-wide `__sx_default_context`. Async runtime
+// is sx LIBRARY code — the compiler provides only the primitives (inline
+// asm, `abi(.naked)`, atomics) + fiber-safe codegen. The protocol is the
+// minimum the fiber scheduler [B1.3+] needs; everything ergonomic
+// (`async` / `await` / `cancel` / `timeout`) is a generic free-fn on top
+// (std/io.sx), the same way `alloc(T,n)` sits over `alloc_bytes`.
+//
+//   spawn_raw  — submit a task; opaque handle (B1.3 fiber bootstrap).
+//   suspend_raw— suspend current fiber; `!` so cancel can raise out.
+//   ready      — wake a parked fiber (B1.4/B1.5).
+//   poll       — drive one step; blocking impl returns 0.
+//   now_ms     — clock hook (a PROTOCOL method so the deterministic-sim
+//                Io [B1.4] can return a fake clock — the B1.4 keystone).
+//   arm_timer  — register a timer; backs `timeout` (B1.4).
+//
+// `ParkToken` is an opaque per-suspension token (unused by the blocking
+// impl). `SpawnOpts.pin` is inert in the M:1 model. (`PinTarget.on` —
+// pin to a specific `Thread` — is deferred with the M:N model; the
+// `on_thread` variant is a placeholder until a `Thread` type exists.)
+PinTarget :: enum { any; main; on_thread; }
+
+SpawnOpts :: struct {
+    pin: PinTarget = .any;
+}
+
+ParkToken :: struct {
+    handle: *void = null;
+}
+
+Io :: protocol #inline {
+    spawn_raw :: (entry: *void, arg: *void, opts: SpawnOpts) -> *void;
+    suspend_raw :: (park: ParkToken) -> !;
+    ready :: (park: ParkToken);
+    poll :: (deadline_ms: i64) -> i64;
+    now_ms :: () -> i64;
+    arm_timer :: (deadline_ms: i64, park: ParkToken) -> *void;
+}
+
 // --- Context ---
+//
+// `allocator` MUST stay at field index 0 (the heap-alloc lowering path
+// hardcodes it — src/ir/lower/call.zig). `io` is appended LAST so `data`
+// keeps its existing index 1 (minimizes the comptime-VM fallback churn).
+// Both Zig materializers of `__sx_default_context` (protocol.zig
+// `emitDefaultContextGlobal` + comptime_vm.zig `materializeDefaultContext`)
+// install the inline `CBlockingIo → Io` vtable at the new field.
 
 Context :: struct {
     allocator: Allocator;
     data: *void;
+    io: Io;
 }
 
 // User-space `xx` extension. `xx val : T` where the built-in conversion

library/modules/std/io.sx

@@ -0,0 +1,135 @@
+// std.io — the `Io` capability's default impl + the async ergonomic layer.
+//
+// `Io` itself (the protocol) lives in std/core.sx next to `Allocator`, so
+// the compiler-coupled `Context` field + the `__sx_default_context`
+// materializers can reference it. This file carries the parts that are
+// pure library sx: the stateless blocking impl (`CBlockingIo`, the mirror
+// of `CAllocator`) + the generic free-fns layered over the protocol
+// (`async` / `await` / `cancel` + the `Future($R)` type).
+//
+// Consumers reach these through std.sx (`Future` / `async` / `await` /
+// `cancel` / `CBlockingIo` re-exports), never by importing this file
+// directly.
+//
+// BLOCKING SEMANTICS (B1.2): the M:1 default has no scheduler and no
+// suspension. `async(worker, ..args)` runs the worker to COMPLETION
+// inline, so the returned `Future` is born `.ready` and `await` yields
+// immediately. `spawn_raw`/`suspend_raw`/`ready`/`poll`/`arm_timer` are
+// trivial no-ops/0 — they exist for the fiber scheduler [B1.3+].
+// `now_ms` returns a real monotonic clock. Fully deterministic/testable.
+//
+// Worker form (B1.2): a `Closure(..$args) -> $R` whose params are
+// annotated at the call site (a lambda `(a: i64) -> i64 => ...`).
+// Named-fn workers need a `::` callable-parameter language feature that
+// does not exist yet and are DEFERRED.
+#import "modules/std/core.sx";
+#import "modules/std/atomic.sx";
+time :: #import "modules/std/time.sx";
+
+// --- IoErr: the error channel async rides (cancellation = model (a)) ---
+//
+// A canceled future raises `.Canceled` out of `await`; a failed task
+// raises `.Failed`. The `(T, !IoErr)` value-failable shape is the same
+// one the rest of the stdlib uses (see examples/1011-, 1012-).
+IoErr :: error { Canceled, Failed }
+
+// --- CBlockingIo: stateless Io that runs tasks synchronously ---
+//
+// Zero-sized struct (mirror of CAllocator). Used as the default
+// `context.io` at program start (see `__sx_default_context` in codegen).
+// The thunks never dereference `self`, so the protocol value's ctx field
+// is `null` — which is what keeps the static-constant default context an
+// inline vtable with a null receiver.
+
+CBlockingIo :: struct {}
+
+impl Io for CBlockingIo {
+    // No fiber bootstrap in the blocking model: the generic `async`
+    // free-fn calls the worker directly and fills the Future. `spawn_raw`
+    // is here for the protocol shape the scheduler [B1.3] will use; the
+    // blocking impl never routes through it, so it is a no-op handle.
+    spawn_raw :: (self: *CBlockingIo, entry: *void, arg: *void, opts: SpawnOpts) -> *void {
+        return null;
+    }
+    // Blocking never suspends — a suspend at the bottom of the M:1 stack
+    // would deadlock. No-op (returns success). The `!` is part of the
+    // protocol contract (a suspending impl raises `.Canceled` out here),
+    // so the conforming blocking impl keeps it even though it never raises.
+    suspend_raw :: (self: *CBlockingIo, park: ParkToken) -> ! {
+        return;
+    }
+    ready :: (self: *CBlockingIo, park: ParkToken) {}
+    poll :: (self: *CBlockingIo, deadline_ms: i64) -> i64 { return 0; }
+    now_ms :: (self: *CBlockingIo) -> i64 { return time.mono_ms(); }
+    arm_timer :: (self: *CBlockingIo, deadline_ms: i64, park: ParkToken) -> *void {
+        return null;
+    }
+}
+
+// --- Future($R): the handle to an async task's eventual result ---
+//
+// Fixed-shape product (NOT the metatype sum machinery). `Value :: $R`
+// exposes the projection `Future(X) → X`. B1.2 supports NON-void `$R`
+// only — `Future(void)` (a `void` struct field) SIGTRAPs the compiler
+// (issue 0150, deferred to B1.4 along with `timeout`).
+FutureState :: enum { pending; ready; failed; canceled; }
+
+Future :: struct ($R: Type) {
+    Value :: R;
+
+    value: R;
+    state: FutureState = .pending;
+    err: IoErr;
+    park: ParkToken;
+    task: *void = null;
+    // Cancellation flag — atomic so a future scheduler thread can flip it.
+    // In the blocking model there is no concurrency, but the type is the
+    // one the M:N model [later] needs.
+    canceled: Atomic(bool);
+}
+
+// --- The async ergonomic layer (generic free-fns over the protocol) ---
+
+// `async(io, worker, ..args)` — submit `worker(..args)`. Blocking: runs
+// the worker to completion inline, Future born `.ready`. The worker is a
+// `Closure(..$args) -> $R` (a lambda whose params are annotated at the
+// call site); `..$args` forwards the call-site arguments to it.
+//
+// NOTE on construction shape: the Future is built with `= ---` + per-field
+// assignment, NOT a `return Future.{...}` struct-literal. A struct-literal
+// in `return` position trips a generic-instantiation gap for the `Atomic`
+// field; the `= ---` (uninit) + field-assign form is the verified idiom.
+async :: ufcs (io: Io, worker: Closure(..$args) -> $R, ..$args) -> Future($R) {
+    f : Future($R) = ---;
+    f.value = worker(..args);
+    f.state = .ready;
+    f.canceled = Atomic(bool).init(false);
+    return f;
+}
+
+// Nullary form — no args. A worker that takes no arguments.
+async_void :: ufcs (io: Io, worker: Closure() -> $R) -> Future($R) {
+    f : Future($R) = ---;
+    f.value = worker();
+    f.state = .ready;
+    f.canceled = Atomic(bool).init(false);
+    return f;
+}
+
+// `await(f)` — value-carrying failable. `.ready` → the result; `.failed`
+// / `.canceled` → raise the stored / cancellation error.
+await :: ufcs (f: *Future($R)) -> ($R, !IoErr) {
+    if f.canceled.load(.acquire) { raise error.Canceled; }
+    if f.state == .canceled      { raise error.Canceled; }
+    if f.state == .failed        { raise error.Failed; }
+    return f.value;
+}
+
+// `cancel(f)` — request cancellation. Sets the per-future cancel flag +
+// marks the state so a subsequent `await` raises `.Canceled`. (In the
+// blocking model the task already ran; cancel still rides the `!`
+// channel — model (a).)
+cancel :: ufcs (f: *Future($R)) {
+    f.canceled.store(true, .release);
+    f.state = .canceled;
+}