035 destructure effect branch when guard

// Destructure + when-guard on an EFFECT branch — the composition of
// 020_018 (! pair { a, b }) and 020_016 (| found { … } when age > 40).
// The guard must see the destructured names, same as on a continuation
// branch. This is the store-watch query shape: projection (destructure)
// + selection (when), both comptime-visible.
//
// COVERAGE RULE (learned the hard way — first draft of this pin had only
// the guarded handler and was correctly rejected KORU022): a required `!`
// branch with only when-guarded handlers is incomplete coverage (ruled,
// pinned 210_085); the rescue is an unguarded fallback sibling (400_089's
// shape, mirrored here).

~import std/io

~pub event scan-pairs {}
! pair { a: i64, b: i64 }
| done

~proc scan-pairs|zig {
    pair(.{ .a = 1, .b = 2 });
    pair(.{ .a = 3, .b = 4 });
    pair(.{ .a = 5, .b = 6 });
    return .done;
}

~scan-pairs()
! pair { a, b } when a > 2 |> std/io:print.ln("big {{ a:d }}+{{ b:d }}")
! pair { a, b } |> std/io:print.ln("small {{ a:d }}+{{ b:d }}")
| done |> std/io:print.ln("done")