088 effect branch when guard struct field

// Test: `when` guard on an `!` effect-branch handler with struct payload,
// guarding on a field access. This is the canonical vaxis-shaped TUI
// dispatch: handle only key events matching a specific character.
//
// The producer fires `! key { ch, code }` for several characters. The
// consumer guards on `k.ch == 'q'` to dispatch only the quit key. A
// catchall for other keys would be the next layer (`!? Transition t |>`)
// but is out of scope for this test — here we pin that:
//   1. `when` works on struct-payload bindings, not just identity.
//   2. Field access in the guard (`k.ch`) parses and lowers correctly.
//   3. Unmatched fires are silently dropped (no handler runs) — the
//      consumer doesn't need to write a sibling handler for the false
//      case of the guard.
//
// Expected output: only the 'q' key fires the handler.

~import std/io

~pub event keystream { count: usize }
! key { ch: u8, code: u8 }

~proc keystream|zig {
    const chars = [_]u8{ 'a', 'b', 'q', 'c' };
    var i: usize = 0;
    while (i < count and i < chars.len) : (i += 1) {
        key(.{ .ch = chars[i], .code = chars[i] });
    }
}

~keystream(count: 4)
! key k when k.ch == 'q' |> std/io:print.blk {
    quit pressed
}
! key _ |> _