215 required effect arm as value keeps handler path

// PINS: a required `!` arm referenced as a VALUE inside a `~proc|zig`
// body keeps the invocation on the handler-call path — `const tick =
// __H.tick` is a real fn value, so the body can store it for later fire.
// The inline splice lowers only `tick(` direct calls; a bare `tick`
// reaches Zig as an undeclared identifier. This is the required-arm
// analogue of 400_168's optional `if (ready) |f| f()` — metal's
// `frames` registers its `! frame` arm on `frame_cb` for the
// live-resize redraw, exactly this shape.
~import std/io

var cb: ?*const fn(i64) void = null;

~pub tor pump {}
! tick i64
| done i64

~proc pump|zig {
    cb = tick;
    if (cb) |f| f(7);
    return .{ .done = 99 };
}

~pump()
! tick t |> std/io:print.ln("tick {{ t:d }}")
| done r |> std/io:print.ln("done {{ r:d }}")