174 effect unguarded arm not last unreachable

// MUST_ERROR — RED BY DESIGN (pinned 2026-07-25).
//
// An unguarded `!` arm that is NOT LAST in an exclusive group silently
// deletes every arm after it. This is `400_089` with the two arms in the
// other order, and it is the trap that bit koru-examples' gallery:
// KORU050 instructs you to add an unguarded else when a branch has
// when-guarded handlers, and WHERE you put that else silently decides
// whether your other arms exist at all.
//
// Today (SHOWN 2026-07-25) this compiles clean, emits `fn tick` containing
// ONLY the unguarded body, and prints small 0..3 — `big` never fires. The
// guarded arm is absent from the artifact, not merely unreachable:
// emitter_helpers.zig:6497 `if (!guarded and !multicast) break;` stops
// emitting the group at the first unguarded arm.
//
// Wanted: a located diagnostic naming the unreachable arms. The natural
// home is flow_checker.zig:181, which already reasons over the guarded /
// unguarded continuation set for KORU050/051 — same pass, same data.
// Error code intentionally NOT asserted here: unassigned pending a ruling
// on whether this joins the KORU05x family or takes a new number.
//
// Deliberately NOT a statement about multicast. Under every dispatch
// policy — exclusive, or a future opt-in fan-out — silently dropping the
// arm is wrong, so this pin holds either way.
~import std/io

~pub tor ticker { n: usize }
! tick usize

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

~ticker(n: 4)
! tick i |> std/io:print.blk {
    small {{ i:d }}
}
! tick i when i > 1 |> std/io:print.blk {
    big {{ i:d }}
}