063 reject single field braces

// Test: a SOLE single-field-brace branch is guided to the bare-return form.
// `| ok { value: i32 }` is a one-variant tag union AND a redundant single-field
// brace. For a lone branch the collapse target is `-> i32` (bare return), NOT
// the identity `| ok i32` (which is itself illegal for a single branch) — so
// the diagnostic must land the user on `-> i32` in one hop, not two. The
// multi-branch single-field case (→ identity) is pinned by 210_144.

const std = @import("std");

~tor compute {}
| ok { value: i32 }