085 when only required effect legal

// Test 210_085: POSITIVE — a `when`-guarded handler covers a REQUIRED
//                         effect branch; no unguarded arm needed.
//
// Was: `reject_when_only_required_effect_branch` — the old doctrine read a
// guarded-only `!` arm as a coverage hole ("fires where the guard is false
// silently produce no work"). The 220_034 ruling reverses that for EFFECTS:
// `!` branches are per-fire filters — `! tick i when i > 1 |> work` means
// "for each fire where the guard holds, work"; unmatched fires are no-ops
// by nature, not dropped coverage. `|` terminals keep the exhaustiveness
// rule (a result must route somewhere — see 210_084).
//
// Companion rulings: `! tick _ |> _` as a SIBLING to a real handler is
// illegal (KORU039 — it discards nothing); a lone discard stays legal as
// explicit handle-and-ignore.
//
// Pins: compiles cleanly AND only guarded fires produce output.
// Expected output: tick 2, done 3.

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

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

~import std/io

~ticker(n: 3)
! tick i when i > 1 |> std/io:print.ln("tick {{ i:d }}")
| done n |> std/io:print.ln("done {{ n:d }}")