081 reject bare terminal residual discard

// ============================================================================
// Test 210_081: MUST_ERROR — bare `|* |> _` residual is pay-for-nothing
//                          and must be parser-rejected.
//
// Without a residual, 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-residual default.
//
// Rule: if you write a residual, 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 residual*.
//
// Expected: parser rejects `|* |> _` with a message pointing at the
// residual line and explaining the no-engagement rule.
// ============================================================================

~tor process { value: u32 }
| success u32
| ?warning string
| ?debug string

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

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