AT-SPI injector: Replace Collection.GetMatches with event-driven or StateSet approach

[Coldaine]

Problem

The AT-SPI text injector currently uses Collection.GetMatches to find focused editable elements, but this method is:

1. Not universally available - The Collection interface isn't implemented by all AT-SPI registries
2. Overkill for the use case - Collection is for complex multi-criteria queries, not simple focus detection
3. Fails on KDE Plasma - Even with at-spi2-core 2.58.1, GetMatches returns UnknownMethod

Error

Collection.get_matches failed: org.freedesktop.DBus.Error.UnknownMethod: 
Method "GetMatches" with signature "(aiia{ss}iaiiasib)uib" on interface 
"org.a11y.atspi.Collection" doesn't exist

Research Findings

Alternative Approaches (simpler and more portable)

1. Event-driven (most efficient):

   listener.register("object:state-changed:focused");
   // Receive ObjectRef of newly focused element directly

2. StateSet querying (most direct):

   let state_set = accessible_proxy.get_state().await?;
   if state_set.contains(State::Focused) {
       // This element is focused
   }

3. Tree walking with state checks:

   async fn find_focused_element(accessible: &AccessibleProxy) -> Option<AccessibleProxy> {
       let state_set = accessible.get_state().await.ok()?;
       if state_set.contains(State::Focused) {
           return Some(accessible.clone());
       }
       // Recursively check children...
   }

KDE Plasma Requirements

Environment variables needed:

export QT_ACCESSIBILITY=1
export QT_LINUX_ACCESSIBILITY_ALWAYS_ON=1

Proposed Solution

1. Replace Collection.GetMatches with event-driven focus tracking
2. Use StateSet queries as fallback
3. Add runtime check for AT-SPI availability before attempting text injection
4. Keep clipboard-based injection as fallback when AT-SPI EditableText unavailable

Affected Code

• crates/coldvox-text-injection/src/injectors/atspi.rs (lines 87-120, 297-330)

Environment

• Nobara Linux 42 (Fedora-based)
• KDE Plasma 6 on Wayland
• at-spi2-core 2.58.1
• Rust atspi crate 0.29

Related

• CI tests now skip AT-SPI when feature not enabled (PR ci: split golden master and hardware integration tests into separate CI jobs #296)
• Original issue discovered while stabilizing real injection tests