fix(cua-driver): bound Linux snapshots after app exit

[aryasaatvik]

Summary

Prevent Linux snapshots from accepting exited or mismatched GUI targets and keep AT-SPI traversal plus geometry resolution within one operation deadline.

Root cause

A zombie remains present under /proc until reaped, so PID existence alone does not prove a process is usable. A PID can also be recycled after exit, making a live-process check insufficient unless the requested X11 window still belongs to that PID. Separately, traversal and bounds collection could each consume timeout budget unless they share one absolute deadline.

Changes

• treat /proc states Z (zombie), X (dead), missing, and malformed status as non-live
• filter non-live windows from list_windows
• reject get_window_state before AT-SPI when the process is non-live or the current X11 window owner does not match the requested PID; revalidate Wayland targets against current enumeration
• use one absolute Tokio deadline across AT-SPI traversal and bounds collection
• add deterministic status parsing, real fork/zombie/reap lifecycle, mismatched-owner, and paused-time shared-budget tests
• add a typed hosted GTK3 X11/AT-SPI regression that snapshots a healthy app, exits it, proves the stale target fails in under one second, and then snapshots a fresh app successfully; bind its behavior recording to the fresh-app recovery snapshot

Integration

The final candidate is rebased onto the merged AT-SPI application-selection fix. The one overlapping test-module conflict was resolved by retaining its duplicate-application and enabled-state coverage alongside this PR's one-deadline traversal/bounds test; production behavior from both changes remains intact.

Validation

Final candidate: d7b44e09a9bed624b075e253889a81ba8f3da69e

• cargo fmt --manifest-path libs/cua-driver/rust/Cargo.toml --all -- --check
• git diff --check
• hosted Linux unit/compile passed at the final candidate, including the deterministic liveness and shared-deadline coverage
• authoritative exact-candidate native run 30912689445 passed:
  • GTK3: 37 passed, 0 failed
  • strict GTK4 target selection: 1 passed, 0 failed
  • typed report: 32 delivered, 7 intentional refusals, 0 failed, 0 skipped
  • linux-gtk3-stale-target-recovery-ax-not-applicable delivered with the AX-state oracle and bound trajectory video
  • no-overlay contract, all videos, and matrix summary passed
• regular PR checks passed for Linux and Windows unit/compile, all hosted Rustfmt targets, generated contract/client parity, release metadata, docs sync, contributor attribution, Nix compositor/package/policy lanes; final Nix Rust unit completion is retained as the last merge gate

Scope

Linux process/window liveness and AT-SPI snapshot budgeting only. This intentionally does not alter AT-SPI application candidate selection or other Linux input/capture behavior.

Authorship

Preserves Saatvik Arya as commit and squash author; adaptation is credited by the existing Francesco coauthor trailer.