feat(editor): M1 pdfrx spike + pen/touch capture

Add pdfrx 2.4.4. PenCaptureRegion routes stylus to ink (arena-bypass
via PenCaptureBinding) while touch falls through to pdfrx scroll/zoom.
Spike pane hosts PdfViewer with page-overlay ink at normalized coords +
frame-time HUD; reachable from home screen for on-device testing.
Bench/coordinate harness under integration_test. Generated assets are
gitignored (regenerate via tool/gen_*.dart).

lib/main.dart

@@ -1,14 +1,22 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:google_fonts/google_fonts.dart';
+import 'package:pdfrx/pdfrx.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
+import 'editor/pdf/pen_capture_region.dart';
 import 'providers/settings_provider.dart';
 import 'screens/home_screen.dart';
 import 'services/database_service.dart';
 
 Future<void> main() async {
-  WidgetsFlutterBinding.ensureInitialized();
+  // Kind-aware binding (extends WidgetsFlutterBinding) must be the active
+  // binding before runApp so the M1 spike's PenCaptureRegion can gate
+  // hit-testing by pointer kind. Safe for the rest of the app: with no pen
+  // region mounted it behaves exactly like the default binding.
+  PenCaptureBinding.ensureInitialized();
+  // pdfrx native engine init (required before any PdfViewer is built).
+  pdfrxFlutterInitialize();
 
   // Ensure DB is ready before the app starts so providers can use it eagerly
   await DatabaseService.getInstance();