081 reject bare terminal catchall discard

// ============================================================================
// Test 210_081: MUST_FAIL — bare `|? |> _` catch-all is pay-for-nothing
//                          and must be parser-rejected.
//
// Without a catch-all, unhandled branches compile-time-elide: the handler
// simply isn't installed in the synthesized comptime struct, so the call
// site lowers to nothing. Zero cost.
//
// With `|? |> _`, the consumer is asking the compiler to *stop eliding*
// unhandled branches, route them into a handler, and then throw the result
// away. That's pay-for-nothing — runtime cost for zero information gain,
// strictly worse than the no-catch-all default.
//
// Rule: if you write a catch-all, you engage. Engagement means the body
// does real work, optionally informed by a metatype binding
// (`|? Transition t |> body`). Anything that lowers to "do nothing on
// unhandled branches" must be expressed by *not writing a catch-all*.
//
// Expected: parser rejects `|? |> _` with a message pointing at the
// catch-all line and explaining the no-engagement rule.
// ============================================================================

~event process { value: u32 }
| success u32
| ?warning []const u8
| ?debug []const u8

~proc process|zig {
    return .{ .success = value * 2 };
}

~process(value: 10)
| success _ |> _
|? |> _