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.

include/ghostty.h

@@ -934,6 +934,7 @@ typedef enum {
   GHOSTTY_ACTION_CONFIG_CHANGE,
   GHOSTTY_ACTION_CLOSE_WINDOW,
   GHOSTTY_ACTION_RING_BELL,
+  GHOSTTY_ACTION_SELECTION_CHANGED,
   GHOSTTY_ACTION_UNDO,
   GHOSTTY_ACTION_REDO,
   GHOSTTY_ACTION_CHECK_FOR_UPDATES,