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/editor/pdf/spike_launcher.dart

@@ -0,0 +1,34 @@
+// lib/editor/pdf/spike_launcher.dart
+//
+// THROWAWAY M1 entry: lets the user open the pdfrx pen/perf spike from the
+// running app (so the CI-built Windows package can exercise MUST #3/#4/#5 on a
+// real Surface Pen with the user's OWN large PDFs). Remove together with the
+// rest of lib/editor/pdf/spike_* once M1 is signed off.
+
+import 'package:file_picker/file_picker.dart';
+import 'package:flutter/material.dart';
+
+import 'spike_editor_pane.dart';
+
+/// Opens a file picker for a PDF, then pushes the spike pane on it.
+/// Pass the user's own large PDF to get a realistic 60fps / pen test.
+Future<void> openM1Spike(BuildContext context) async {
+  final result = await FilePicker.platform.pickFiles(
+    type: FileType.custom,
+    allowedExtensions: ['pdf'],
+  );
+  final path = result?.files.single.path;
+  if (path == null) return;
+  if (!context.mounted) return;
+  Navigator.of(context).push(
+    MaterialPageRoute(
+      builder: (_) => Scaffold(
+        appBar: AppBar(title: const Text('M1 Spike — pen / scroll / zoom')),
+        // denseStrokesAsset is null here: the bundled synthetic-stroke load is
+        // only for the perf bench. For manual on-device testing, draw real ink
+        // and scroll a real large PDF while watching the frame-time HUD.
+        body: SpikeEditorPane(pdfPath: path),
+      ),
+    ),
+  );
+}