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/screens/home_screen.dart

@@ -5,6 +5,7 @@ import '../models/note.dart';
 import '../providers/document_provider.dart';
 import '../providers/note_provider.dart';
 import '../providers/ocr_provider.dart';
+import '../editor/pdf/spike_launcher.dart';
 import '../services/pdf_service.dart';
 import '../services/pptx_service.dart';
 import 'note_editor_screen.dart';
@@ -67,6 +68,12 @@ class HomeScreen extends ConsumerWidget {
               ).push(MaterialPageRoute(builder: (_) => const SearchScreen()));
             },
           ),
+          // THROWAWAY M1 spike entry (remove with lib/editor/pdf/spike_*).
+          IconButton(
+            icon: const Icon(Icons.science_outlined),
+            tooltip: 'M1 Spike (pen/scroll/zoom test)',
+            onPressed: () => openM1Spike(context),
+          ),
         ],
       ),
       floatingActionButton: FloatingActionButton(