feat: sync computer-use-linux v0.4.9

[avifenesh]

Summary

• sync the embedded Computer Use backend from v0.4.7 to the released v0.4.9 implementation
• preserve the downstream 0.4.9-linux-alpha1 package and MCP identity
• add bounded, indexed AT-SPI traversal with fixed discovery/read budgets, bounded concurrency, clamped public limits, and a snapshot deadline
• carry the newer Linux defaults and document the accumulated X11, KDE, GTK4, and traversal improvements

Standalone release: https://github.com/agent-sh/computer-use-linux/releases/tag/v0.4.9
Source fixes: agent-sh/computer-use-linux#91
Release PR: agent-sh/computer-use-linux#92

Validation

• cargo fmt --all -- --check
• cargo test -p codex-computer-use-linux -- --test-threads=1 (all targets and doctests)
• cargo clippy -p codex-computer-use-linux --all-targets -- -D warnings
• CI_SKIP_PULL=1 ./scripts/ci-local.sh deb
• live Nautilus CLI snapshot: 606 nodes, depth 29, completed in about one second
• live MCP initialization reports codex-computer-use-linux 0.4.9-linux-alpha1
• live MCP get_app_state with max_nodes=1 returns exactly one Nautilus accessibility node
• independent code-quality, backend, security, performance, and test-coverage reviews: clean

[ilysenko]

Reviewed the complete base-to-head diff. The v0.4.9 sync is focused, bounded, and covered by the relevant green checks.