170 effect inline unhandled void arm discard

// 400_170: an UNHANDLED VOID optional arm fired by direct call on the inline
// path lowers to a clean no-op. Under Option B (2026-07-19) a yielding arm is
// invoked directly (`tick()`); when the consumer omits the handler, the inline
// rewriter drops the call. For a VOID arm there is no payload to evaluate, so it
// must emit `_ = {}` — NOT the invalid `_ = ()` (the payload-discard shape).
//
// Green here proves the void-discard path; a regression would be Zig
// "expected expression, found ')'". (Surfaced by koru-libs vaxis: an app that
// omits `! focus-in` / `! focus-out`.)
~import std/io

~pub tor beat {}
! ?tick
| done i64

~proc beat|zig {
    tick();
    tick();
    return .{ .done = 2 };
}

~beat()
| done r |> std/io:print.ln("done {{ r:d }}")