087 effect branch when guard

// Test: `when` guard on an `!` effect-branch handler at the dispatch site.
//
// The producer fires `! tick(i)` for i in 0..5. The consumer handles only
// the even values via a `when` guard on the binding. This pins:
//   1. The parser accepts `! name binding when cond |> body`.
//   2. The guard is preserved through the comptime-struct synthesis — the
//      lowered handler must contain the conditional, not the unguarded
//      body. (Otherwise every i would print, not just the even ones.)
//   3. Branches-are-equal: a `when`-guarded `!` handler is just a `!`
//      handler with a condition, no special exhaustiveness rules apply.
//
// Expected output: even values only (0, 2, 4).

~import std/io

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

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

~ticker(n: 5)
! tick i when i % 2 == 0 |> std/io:print.blk {
    tick {{ i:d }}
}
! tick _ |> _