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_app.dart

@@ -0,0 +1,187 @@
+// lib/editor/pdf/spike_app.dart
+//
+// THROWAWAY M1 spike app shell (plan §10). Wraps [SpikeEditorPane] with an
+// on-screen frame-timing HUD (median build & raster ms over the last ~120
+// frames) and an ink-load toggle, so MUST #4/#5 are observable on-device when
+// launched via `flutter run -t lib/editor/pdf/spike_main.dart` on the tablet.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:pdfrx/pdfrx.dart';
+
+import 'spike_editor_pane.dart';
+
+class SpikeApp extends StatelessWidget {
+  const SpikeApp({super.key, required this.pdfPath, this.denseStrokesAsset});
+
+  final String pdfPath;
+  final String? denseStrokesAsset;
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'BadNote M1 Spike',
+      debugShowCheckedModeBanner: false,
+      theme: ThemeData(useMaterial3: true, colorSchemeSeed: Colors.indigo),
+      home: SpikeHome(
+        pdfPath: pdfPath,
+        denseStrokesAsset: denseStrokesAsset,
+      ),
+    );
+  }
+}
+
+class SpikeHome extends StatefulWidget {
+  const SpikeHome({super.key, required this.pdfPath, this.denseStrokesAsset});
+
+  final String pdfPath;
+  final String? denseStrokesAsset;
+
+  @override
+  State<SpikeHome> createState() => _SpikeHomeState();
+}
+
+class _SpikeHomeState extends State<SpikeHome> {
+  final GlobalKey<SpikeEditorPaneState> _paneKey =
+      GlobalKey<SpikeEditorPaneState>();
+  final PdfViewerController _controller = PdfViewerController();
+  bool _inkLoad = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Stack(
+        children: [
+          SpikeEditorPane(
+            key: _paneKey,
+            controller: _controller,
+            pdfPath: widget.pdfPath,
+            denseStrokesAsset: widget.denseStrokesAsset,
+          ),
+          const Positioned(top: 8, left: 8, child: FrameTimingHud()),
+        ],
+      ),
+      floatingActionButton: Column(
+        mainAxisSize: MainAxisSize.min,
+        crossAxisAlignment: CrossAxisAlignment.end,
+        children: [
+          FloatingActionButton.extended(
+            heroTag: 'inkload',
+            onPressed: () async {
+              final next = !_inkLoad;
+              await _paneKey.currentState?.setInkLoad(next);
+              setState(() => _inkLoad = next);
+            },
+            label: Text(_inkLoad ? 'Ink load: ON' : 'Ink load: OFF'),
+            icon: const Icon(Icons.brush),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+/// On-screen median build/raster frame-time HUD, driven by
+/// [SchedulerBinding.addTimingsCallback]. Shows the median of the last
+/// [_window] frames for both the build (`buildDuration`) and raster
+/// (`rasterDuration`) phases — the two halves of the 16.6ms budget tracked by
+/// MUST #4/#5.
+class FrameTimingHud extends StatefulWidget {
+  const FrameTimingHud({super.key});
+
+  @override
+  State<FrameTimingHud> createState() => _FrameTimingHudState();
+}
+
+class _FrameTimingHudState extends State<FrameTimingHud> {
+  static const int _window = 120;
+  final List<double> _build = <double>[];
+  final List<double> _raster = <double>[];
+  double _medBuild = 0;
+  double _medRaster = 0;
+  double _p95Build = 0;
+  double _p95Raster = 0;
+
+  @override
+  void initState() {
+    super.initState();
+    SchedulerBinding.instance.addTimingsCallback(_onTimings);
+  }
+
+  @override
+  void dispose() {
+    SchedulerBinding.instance.removeTimingsCallback(_onTimings);
+    super.dispose();
+  }
+
+  void _onTimings(List<FrameTiming> timings) {
+    for (final t in timings) {
+      _build.add(t.buildDuration.inMicroseconds / 1000.0);
+      _raster.add(t.rasterDuration.inMicroseconds / 1000.0);
+    }
+    while (_build.length > _window) {
+      _build.removeAt(0);
+    }
+    while (_raster.length > _window) {
+      _raster.removeAt(0);
+    }
+    if (!mounted) return;
+    setState(() {
+      _medBuild = _percentile(_build, 50);
+      _medRaster = _percentile(_raster, 50);
+      _p95Build = _percentile(_build, 95);
+      _p95Raster = _percentile(_raster, 95);
+    });
+  }
+
+  static double _percentile(List<double> values, int p) {
+    if (values.isEmpty) return 0;
+    final sorted = List<double>.from(values)..sort();
+    final idx = ((p / 100.0) * (sorted.length - 1)).round();
+    return sorted[idx.clamp(0, sorted.length - 1)];
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    Color budget(double ms) => ms <= 16.6
+        ? Colors.greenAccent
+        : (ms <= 22 ? Colors.amberAccent : Colors.redAccent);
+    return IgnorePointer(
+      child: Container(
+        padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 6),
+        decoration: BoxDecoration(
+          color: Colors.black.withValues(alpha: 0.65),
+          borderRadius: BorderRadius.circular(8),
+        ),
+        child: DefaultTextStyle(
+          style: const TextStyle(
+            fontFamily: 'monospace',
+            fontSize: 12,
+            color: Colors.white,
+          ),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Text('frames: ${_build.length}/$_window'),
+              Text.rich(TextSpan(children: [
+                const TextSpan(text: 'build  med '),
+                TextSpan(
+                    text: '${_medBuild.toStringAsFixed(1)}ms',
+                    style: TextStyle(color: budget(_medBuild))),
+                TextSpan(text: '  p95 ${_p95Build.toStringAsFixed(1)}ms'),
+              ])),
+              Text.rich(TextSpan(children: [
+                const TextSpan(text: 'raster med '),
+                TextSpan(
+                    text: '${_medRaster.toStringAsFixed(1)}ms',
+                    style: TextStyle(color: budget(_medRaster))),
+                TextSpan(text: '  p95 ${_p95Raster.toStringAsFixed(1)}ms'),
+              ])),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}