fibers B1.2: BLOCKED on compiler bugs 0150 + 0151 (Io design proven)

Stream B1 B1.2 (Io capability + context.io + Future + cancel) is blocked on
two newly-discovered, independent compiler bugs, both with standalone repros:

- 0150: a `void` struct field crashes the compiler with an unsized-type
  SIGTRAP in LLVM getTypeSizeInBits. Blocks `Future(void)` -> `timeout`.
- 0151: a type-var inferred from a fn-pointer parameter's RETURN type is not
  bound as a usable type in the function body (`unknown type 'R'`). Blocks the
  central `async(io, worker: ($A)->$R, arg)` free-fn's `Future(R)`.

The B1.2 design itself is validated end-to-end (the Io protocol threaded on
Context like Allocator, the stateless blocking CBlockingIo default, both
__sx_default_context materializers, and `context.io.now_ms()` all work live).
Only the async/await/timeout ergonomic layer hits the two bugs. Per the
IMPASSABLE STOP rule, all B1.2 working changes were reverted (master green,
726/0) and the work paused pending fixes; WIP is saved at .sx-tmp/b12-wip/.

Checkpoint + plan updated to mark B1.2 BLOCKED with full resume notes.

issues/0150-void-struct-field-unsized-llvm-trap.sx

@@ -0,0 +1,13 @@
+// Repro for issue 0150 — a `void` struct field crashes the compiler with an
+// unsized-type SIGTRAP (LLVM getTypeSizeInBits). Unpinned (no expected marker)
+// because it currently aborts the compiler; pin it as a regression test once
+// the fix lands.
+#import "modules/std.sx";
+
+Holder :: struct { v: void; ok: bool; }
+
+main :: () -> i32 {
+    h : Holder = .{ ok = true };
+    if h.ok { print("ok\n"); }
+    return 0;
+}