feat: add useIsScreenReaderEnabled + MountOptions.isScreenReaderEnabled

- Default: true when INK_SCREEN_READER=true env var set
- Screen reader mode bypasses render throttle
- Composable exposes flag via provide/inject

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/runtime/src/render-to-string.ts

@@ -157,6 +157,7 @@ function createNoOpAppContext(): AppContext {
     stdin: process.stdin,
     debug: false,
     interactive: false,
+    isScreenReaderEnabled: false,
     isRawModeSupported: false,
     setRawMode: () => {},
     writeToStdout: () => {},