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/lower.zig

@@ -391,6 +391,14 @@ pub const Lowering = struct {
     /// escape tags in. Read by `checkEscapeWidening` when a `try` operand is a
     /// closure/fn-type SLOT call (no static fn name). Key = `closureShapeKey`.
     shape_inferred_sets: std.StringHashMap([]const u32),
+    /// Qualified names (`Type.method`) of every explicitly-written protocol
+    /// impl method. A protocol method may be declared `!` (the error channel
+    /// is part of the contract — e.g. `Io.suspend_raw`); a conforming impl
+    /// MUST keep the `!` even when its concrete body never raises, so the
+    /// "declared `!` but never errors — drop the `!`" warning (a free-fn
+    /// linting hint) is a false positive for these. The empty-inferred-set
+    /// warning in `error_analysis.zig` skips names in this set.
+    impl_method_names: std.StringHashMap(void),
 
     pub const ComptimeValue = union(enum) {
         int_val: i64,
@@ -536,6 +544,7 @@ pub const Lowering = struct {
             .comptime_constants = std.StringHashMap(ComptimeValue).init(module.alloc),
             .xx_reentrancy = std.AutoHashMap(u64, void).init(module.alloc),
             .inferred_error_sets = std.StringHashMap([]const u32).init(module.alloc),
+            .impl_method_names = std.StringHashMap(void).init(module.alloc),
             .shape_inferred_sets = std.StringHashMap([]const u32).init(module.alloc),
             .program_index = ProgramIndex.init(module.alloc),
         };