153 effect lone optional

// Test 400_153: a LONE optional effect branch — `! ?warn` as the event's only
// branch. Positive coverage of the "single optional effect arm" grid cell.
//
// `! ?warn []const u8` carries a payload, so the redundant-no-payload rule does
// NOT apply; it parses as an OPTIONAL EFFECT branch named "warn" and runs. The
// `?` modifier is stripped on the lone-branch path exactly as with siblings
// (cf. 210_076) — the single-branch case does NOT diverge.
//
// Dispatch handles the arm by NAME (`! warn w`): the `?` is a declaration
// property, not repeated at the call site.
//
// DESIGN (effect vs continuation, ruled 2026-06-27): effect branches are flexible
// — 0, a lone arm (void or payload, plain or `?`-optional), or many — and the
// `?`/`when` modifiers apply uniformly regardless of count.
~import std/io

~pub event maybe { n: usize }
! ?warn []const u8

~proc maybe|zig {
    var i: usize = 0;
    while (i < n) : (i += 1) { warn("beep"); }
}

~maybe(n: 2)
! warn w |> std/io:print.blk {
    {{ w:s }}
}