perf(ax-tree): AX attribute name CFStrings are re-allocated on every tree traversal

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Problem

crates/tday-nativecore/src/platform/macos/ax.rs L507, L514–515

Helper functions like ax_children, get_string, get_bool, and get_ax_value allocate a new CFString on every call:

unsafe fn get_string(el: AXUIElementRef, attr_name: &str) -> Option<String> {
    let attr = CFString::new(attr_name); // ← allocates on every call
    ...
}

A 10,000-node AX tree snapshot calls get_string/get_bool 5–8 times per node, totalling ~50,000–80,000 CFString heap allocations per snapshot.

Suggested fix

Declare commonly used attribute names (AXRole, AXTitle, AXValue, AXDescription, AXEnabled, AXFocused, AXChildren, AXPosition, AXSize) as global lazy_static or once_cell::sync::Lazy<CFString> constants and reuse them for the lifetime of the process.