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_main.dart

@@ -0,0 +1,62 @@
+// lib/editor/pdf/spike_main.dart
+//
+// Standalone entry point for the THROWAWAY M1 pdfrx spike (plan §10).
+//
+// Launch on the Windows tablet (or any desktop with a display):
+//   flutter run -t lib/editor/pdf/spike_main.dart
+//
+// It opens test/assets/large_300p.pdf in [SpikeEditorPane] with the
+// frame-timing HUD and ink-load toggle, so the M1 perf/pen gates are
+// observable on-device.
+//
+// IMPORTANT: pen capture requires the kind-aware [PenCaptureBinding] (installed
+// below before pdfrx init). pdfrx itself is initialized via
+// pdfrxFlutterInitialize() — confirmed from pdfrx 2.4.4 example/pdf_combine.
+
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'package:pdfrx/pdfrx.dart';
+
+import 'pen_capture_region.dart';
+import 'spike_app.dart';
+
+/// Default benchmark asset (300-page PDF generated by tool/gen_bench_pdf.dart).
+const String _kDefaultPdfRelPath = 'test/assets/large_300p.pdf';
+
+/// Filesystem path for the synthetic ink load (regenerate via
+/// tool/gen_dense_strokes.dart; not bundled — read from disk at the project root).
+const String _kDenseStrokesAsset = 'test/assets/dense_strokes.json';
+
+void main(List<String> args) {
+  // Kind-aware binding MUST be installed before runApp so PenCaptureRegion can
+  // gate hit-testing by pointer kind (see pen_capture_region.dart header).
+  PenCaptureBinding.ensureInitialized();
+  // pdfrx native engine init (pdfrx 2.4.4 example pattern).
+  pdfrxFlutterInitialize();
+
+  // Allow overriding the PDF path as the first CLI arg (otherwise the default
+  // 300-page bench asset relative to the project root / cwd).
+  final pdfPath = args.isNotEmpty ? args.first : _resolvePdfPath();
+
+  runApp(
+    SpikeApp(
+      pdfPath: pdfPath,
+      denseStrokesAsset: _kDenseStrokesAsset,
+    ),
+  );
+}
+
+/// Resolve the bench PDF path. `flutter run` sets cwd to the project root, so
+/// the relative asset path works on desktop; we also try a couple of fallbacks.
+String _resolvePdfPath() {
+  final candidates = <String>[
+    _kDefaultPdfRelPath,
+    '${Directory.current.path}/$_kDefaultPdfRelPath',
+  ];
+  for (final c in candidates) {
+    if (File(c).existsSync()) return c;
+  }
+  // Return the primary path anyway; pdfrx will surface a clear load error.
+  return _kDefaultPdfRelPath;
+}