097 required terminal must be handled

// Negative test: consumer must handle every non-optional terminal of an
// event. The `?` modifier on a terminal (`| ?done`) marks it as optional
// — consumer may omit; the compiler synthesizes a no-op switch arm. Without
// the `?`, the terminal is required, and omitting the handler must error.
//
// The event below declares `| done` (required) and `| err []const u8`
// (required). The consumer's flow handles `| err` but NOT `| done`. The
// compiler must reject this — silent auto-synthesis of the missing arm
// would mean `?` carries no meaning and the language has no way to
// distinguish "optional terminal" from "you forgot a handler".

~import std/io

~pub event run {}
| done
| err []const u8

~proc run|zig {
    return .{ .done = .{} };
}

~run()
| err msg |> std/io:print.ln(msg)