161 prototype hole rejected without annotation

// The production guarantee behind prototype mode: WITHOUT `~[prototype]`, an
// unhandled non-optional terminal branch is rejected KORU022 — exactly as
// today (400_097). This is the SAME source as 400_160 with the annotation
// removed; the annotation is the only thing that lets the hole compile.
//
// This pins that prototype mode cannot leak into a production build: a hole is
// a hard compile error unless the file explicitly opts in with `~[prototype]`.
// There is no flag a caller can pass to silence it — the opt-in lives in the
// source, per-file, greppable.

~import std/io

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

~proc run|zig {
    return .{ .err = "built path ran; done is still a hole" };
}

~run()
| err msg |> std/io:print.ln(msg)
// `| done` unhandled and NO `~[prototype]` — must fail KORU022.