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/error_analysis.zig

@@ -174,7 +174,11 @@ pub const ErrorAnalysis = struct {
             const sorted = self.l.alloc.dupe(u32, se.value_ptr.tags.items) catch continue;
             std.mem.sort(u32, sorted, {}, std.sort.asc(u32));
             self.l.inferred_error_sets.put(se.key_ptr.*, sorted) catch {};
-            if (sorted.len == 0 and !std.mem.eql(u8, se.key_ptr.*, "main")) {
+            // Skip `main` (its `!` is the program's top error channel) and any
+            // protocol-impl method (its `!` is dictated by the protocol
+            // contract — e.g. `Io.suspend_raw` — so a non-raising impl body
+            // is not a "drop the `!`" case; see `impl_method_names`).
+            if (sorted.len == 0 and !std.mem.eql(u8, se.key_ptr.*, "main") and !self.l.impl_method_names.contains(se.key_ptr.*)) {
                 if (self.l.diagnostics) |diags| {
                     if (se.value_ptr.rt) |rt| {
                         diags.addFmt(.warn, rt.span, "function '{s}' is declared `!` but never errors — drop the `!`", .{se.key_ptr.*});