core: send selection_changed notification

The core had no signal to the apprt when the active selection changed,
so a consumer (e.g. a screen reader) kept reading a stale selection
until some unrelated query refreshed it.

This change adds a payload-less selection_changed action that's fired on
a selection state transition. The apprt reads the current selection
through the normal read path.

This consolidates selection state changes so the notification fires
consistently: all sites route through setSelection rather than calling
screen.select directly, including the mouse paths that previously
bypassed it for clipboard timing.

The new setSelectionAndCopy extends setSelection with the additional
'copy_on_select' behavior.

On macOS, this posts .ghosttySelectionDidChange, which is debounced
before posting a NSAccessibility .selectedTextChanged notification.

GTK has no consumer yet and no-ops the action.

src/apprt/action.zig

@@ -295,6 +295,10 @@ pub const Action = union(Key) {
     /// it needs to ring the bell. This is usually a sound or visual effect.
     ring_bell,
 
+    /// Called when the active selection changes. The apprt should read the
+    /// current selection itself; this carries no payload.
+    selection_changed,
+
     /// Undo the last action. See the "undo" keybinding for more
     /// details on what can and cannot be undone.
     undo,
@@ -396,6 +400,7 @@ pub const Action = union(Key) {
         config_change,
         close_window,
         ring_bell,
+        selection_changed,
         undo,
         redo,
         check_for_updates,

src/apprt/gtk/class/application.zig

@@ -743,6 +743,9 @@ pub const Application = extern struct {
 
             .ring_bell => Action.ringBell(target),
 
+            // GTK has no accessibility consumer for this yet.
+            .selection_changed => {},
+
             .scrollbar => Action.scrollbar(target, value),
 
             .set_title => Action.setTitle(target, value),