fix(cua-driver-rs)(macos): per-element AX messaging timeout + cycle/time guards for heavy webview walks

[f-trycua]

Problem

get_window_state can hang indefinitely on apps with large or busy accessibility trees (heavy webviews). Reading an AX attribute is a synchronous XPC round-trip into the target app; while a renderer is materializing its tree, that round-trip can stay pending forever and wedge the entire walk. The outer thread-level timeout then just tears down a stuck walk rather than letting it make progress.

Fix

Set a per-element messaging timeout once, on the application element, right after AXUIElementCreateApplication:

• AXUIElementSetMessagingTimeout(app_elem, 1.5) — the ceiling is inherited by every descendant element vended through the app element, so a single wedged element fails fast with kAXErrorCannotComplete in ~1.5s instead of blocking the whole traversal. This is the real anti-hang fix.

Two layered backstops in walk_element:

• Cycle dedup — a visited: HashSet<usize> keyed on the element pointer address. AX is a graph, not a strict tree; a malformed hierarchy can re-vend an element already walked, and this stops that from inflating the walk.
• 5s per-walk wall-clock budget — bails between AX calls the same way the depth guard does. It cannot abort an in-flight call (that is the messaging timeout's job); it caps cumulative time across many slow-but-completing calls.

These complement, not replace, the existing outer thread-level timeout and the MAX_DEPTH guard.

Scope

• platform-macos only; platform-windows / platform-linux unaffected.
• AXUIElementSetMessagingTimeout is a stable HIServices symbol (since 10.4); declared alongside the other AXUIElement* FFI bindings.

Validation

• cargo build --release -p cua-driver passes.
• cargo test --no-run -p platform-macos compiles (the pre-existing libswift_Concurrency.dylib rpath issue is at the test-binary link step, not introduced here).
• Live VM validation against heavy webview apps still recommended before un-drafting.

Refs #1537

🤖 Generated with Claude Code

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	May 29, 2026 4:35pm