152 effect void beat heartbeat

// Test 400_152: a LONE payloadless effect branch — a "heartbeat".
//
// `! beat` is the event's only branch: no payload, no resume, no terminal. It
// fires for the fact of firing. This is the simplest void effect — the mouse-
// move / keypress / tick shape — and it is NOT redundant because it MULTIFIRES:
// the consumer body (`|> print thump`) runs once per beat, which calling the
// proc directly cannot reproduce.
//
// DESIGN (effect vs continuation surfaces, ruled 2026-06-27): the redundant-
// single-branch rejection (PARSE003) is a CONTINUATION (`|`) rule — a lone empty
// terminal carries nothing and is genuinely redundant. It must NOT apply to an
// EFFECT (`!`) arm, which is a yield point. Effect branches allow {0, a single
// payloadless arm, many}; continuation branches do not.
//
// Currently RED: PARSE003 rejects this lone `! beat`. Pinned MUST_RUN to drive
// scoping that rejection to terminal branches only.
~import std/io

~pub event heartbeat { n: usize }
! beat

~proc heartbeat|zig {
    var i: usize = 0;
    while (i < n) : (i += 1) { beat(); }
}

~heartbeat(n: 3)
! beat |> std/io:print.ln("thump")