perf(native): use index queues for AX BFS

[JustYannicc]

What changed

• Replaced Array.removeFirst() queue consumption in the native Accessibility BFS loops with monotonic index cursors.
• Added scripts/bench-ax-bfs-queue.swift so the queue behavior benchmark is repeatable.

Why

Accessibility selected-text and caret lookup run on hot paths. Array.removeFirst() shifts/copies the remaining queue on every pop, even though these traversals only need FIFO order and already cap depth/element counts.

Behavior and compatibility

• Preserves BFS search order, focused-child preference, depth limits, and selected/caret lookup fallback order.
• No user-facing strings changed.

Evidence

• Baseline before editing, synthetic Swift BFS queue benchmark: removeFirst: 235.47 ms, cursor: 37.95 ms, equal checksum, 6.20x speedup.
• Repeatable benchmark after change: swift -O scripts/bench-ax-bfs-queue.swift -> removeFirst: 211.29 ms, cursor: 36.80 ms, equal checksum, 5.74x speedup.
• LSP diagnostics: no errors for src/native/ax-caret-query.swift, src/native/get-selected-text.swift, and scripts/bench-ax-bfs-queue.swift.
• swiftc -O -o /tmp/supercmd-get-selected-text-check src/native/get-selected-text.swift -framework Foundation -framework ApplicationServices -framework AppKit.
• swiftc -O -o /tmp/supercmd-emoji-trigger-monitor-check src/native/emoji-trigger-monitor.swift src/native/ax-caret-query.swift -framework AppKit -framework ApplicationServices.
• node --test 'scripts/test-*.mjs' passed: 24 passed, 1 skipped.
• node scripts/check-i18n.mjs exited 0; it reports pre-existing Italian locale gaps unrelated to this native-only change.

Risk

Low. The traversal queue still appends in the same order and stops at the same caps; the only intentional change is avoiding per-pop array shifting.

[JustYannicc]

Superseded by consolidated upstream PR #534 (perf(mac): reduce native polling and async IO stalls), which carries this change together with the related macOS/native/audio/camera async IO performance fixes. Closing this source PR so review can continue on #534.