fix: bound AT-SPI traversal work

[avifenesh]

Summary

• bound queued AT-SPI references and indexed child-read attempts across registry discovery, root filtering, snapshots, and focus probes
• replace eager GetChildren allocations with ordered, bounded-concurrency indexed reads
• clamp the public snapshot boundary and cap snapshots with a 10-second deadline
• preserve explicit registry failures instead of reporting a successful empty tree

Validation

• cargo test --locked --no-fail-fast -- --test-threads=1 (249 tests)
• cargo clippy --locked --all-targets -- -D warnings
• python3 scripts/mcp_safety_check.py --binary target/debug/computer-use-linux
• node scripts/zod-check/check.mjs --command target/debug/computer-use-linux
• live Nautilus state: 606 raw nodes, depth 29, 1.2s
• live MCP get_app_state with max_nodes=1: one Nautilus node returned
• iterative code-quality, security, performance, coverage, and Rust/backend review: clean