fix(textinput): announce caret navigation to screen readers

The Composition WindowsTextInput never raised a UI Automation
TextSelectionChanged event when the caret or selection moved, so
Narrator and braille displays stayed silent while arrowing through
text. Focus was announced on entering the field, but per-character,
word, and line caret navigation produced no event for assistive
technology to re-read.

Raise UIA_Text_TextSelectionChangedEventId from OnSelectionChanged,
guarded by UiaClientsAreListening(), mirroring the existing
focus-changed and value-changed raises elsewhere in the Composition
layer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ClintC

change/react-native-windows-b55776cc-2475-4854-b426-d55aed0fddb3.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Raise a UIA TextSelectionChanged event when the TextInput caret/selection moves, so Narrator and braille displays announce character/word/line navigation during keyboard caret movement",
+  "packageName": "react-native-windows",
+  "email": "clintc@microsoft.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Fabric/Composition/TextInput/WindowsTextInputComponentView.cpp

@@ -1457,6 +1457,17 @@ void WindowsTextInputComponentView::OnSelectionChanged(LONG start, LONG end) noe
     onSelectionChangeArgs.selection.end = end;
     emitter->onSelectionChange(onSelectionChangeArgs);
   }
+
+  // Notify UI Automation clients (e.g. Narrator and braille displays) that the caret or
+  // selection moved, so they re-read the current character/word/line during keyboard
+  // navigation. Without this, arrowing through the text is silent for screen readers.
+  // Mirrors the focus- and value-changed raises elsewhere in the Composition layer.
+  if (UiaClientsAreListening()) {
+    auto spProviderSimple = EnsureUiaProvider().try_as<IRawElementProviderSimple>();
+    if (spProviderSimple != nullptr) {
+      UiaRaiseAutomationEvent(spProviderSimple.get(), UIA_Text_TextSelectionChangedEventId);
+    }
+  }
 }
 
 std::string WindowsTextInputComponentView::GetTextFromRichEdit() const noexcept {