feat: AT-SPI focus backend implementation

[Coldaine]

Source

PR #232 () - can be closed after this issue exists.

What it does

Real AT-SPI focus detection replacing the stub SystemFocusAdapter. Uses the atspi crate to query the currently focused UI element and determine if it's an editable text field.

Key code (~100 lines)

#[cfg(feature = "atspi")]
#[async_trait]
impl FocusBackend for SystemFocusAdapter {
    async fn query_focus(&self) -> Result<FocusStatus, InjectionError> {
        use atspi::{
            connection::AccessibilityConnection,
            proxy::accessible::AccessibleProxy,
            proxy::collection::CollectionProxy,
            Interface, MatchType, ObjectMatchRule, SortOrder, State,
        };

        let conn = match AccessibilityConnection::new().await {
            Ok(c) => c,
            Err(e) => {
                log_atspi_connection_failure(&e.to_string());
                return Ok(FocusStatus::Unknown);
            }
        };

        let zbus_conn = conn.connection();
        let collection = CollectionProxy::builder(zbus_conn)
            .destination("org.a11y.atspi.Registry")?
            .path("/org/a11y/atspi/accessible/root")?
            .build().await?;

        let mut rule = ObjectMatchRule::default();
        rule.states = State::Focused.into();
        rule.states_mt = MatchType::All;

        let matches = collection.get_matches(rule, SortOrder::Canonical, 1, false).await?;

        if let Some(obj_ref) = matches.first() {
            let accessible = AccessibleProxy::builder(zbus_conn)
                .destination(obj_ref.name.clone())?
                .path(obj_ref.path.clone())?
                .build().await?;

            let ifaces = accessible.get_interfaces().await?;
            if ifaces.contains(Interface::EditableText) {
                Ok(FocusStatus::EditableText)
            } else {
                Ok(FocusStatus::NonEditable)
            }
        } else {
            Ok(FocusStatus::Unknown)
        }
    }
}

Implementation notes

• Feature-gated behind atspi feature flag
• Graceful fallback to FocusStatus::Unknown on errors
• Needs runtime testing on GNOME/GTK desktop
• Test file included: focus_backend_test.rs (gated by live-hardware-tests)

Status

Ready to implement fresh or cherry-pick from PR #232 after rebase.