166 undeclared branch rejected without annotation

// The production guarantee behind the undeclared-branch relaxation (400_165):
// WITHOUT `~[prototype]`, a continuation arm for a branch the event does not
// declare is rejected KORU021 (unknown branch in continuation) — exactly as
// today. This is the SAME source as 400_165 with the annotation removed; the
// annotation is the only thing that lets the undeclared arm slide.
//
// This pins that the prototype doodle cannot leak into a production build: an
// undeclared arm is a hard compile error unless the file explicitly opts in
// with `~[prototype]`. "Lift the tag and it fails loudly" — the arm you thought
// about but never declared becomes an error the moment prototype is off.

~import std/io

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

~proc run|zig {
    return .{ .err = "declared path ran" };
}

~run()
| done |> std/io:print.ln("done ran")
| err msg |> std/io:print.ln(msg)
| soon |> std/io:print.ln("undeclared arm — KORU021 without ~[prototype]")