feat(pen): forked InteractiveViewer (stylus-exclusive draw) + zoom diagnostic

Replace stock InteractiveViewer with PenInteractiveViewer, a focused fork of
Flutter 3.44's InteractiveViewer for our config (constrained=false, infinite
boundary, no rotation — that machinery dropped as a no-op here). Two deliberate
changes, grounded in the Rnote/Saber research:

1. The pan/zoom ScaleGestureRecognizer excludes stylus/invertedStylus via
   `supportedDevices`. The pen never reaches it, so a stylus stroke can no longer
   be stolen as a pan on its first frame (the "写字识别成单击" feel bug, caused by
   stock IV's panEnabled updating a frame after the stroke began). Drawing is
   owned solely by the canvas Listener; no arena fight, no panEnabled lag. The
   prior _lastStylus hover hack is removed (superseded).

2. Per-frame scale change is clamped (×0.74..×1.35). Stock IV already damps focal
   jitter and guards the pan branch, but a single-frame multi-touch glitch could
   still spike details.scale, popping the zoom bigger/smaller and snapping back
   (the reported pinch flicker). Clamping swallows the spike; a real (gradual)
   pinch is unaffected since scale tracks absolutely from gesture start. Cap is
   far above any real pinch (~1.1-1.2x/frame), so no felt lag.

Everything else (scale-about-focal, pan, fling inertia, mouse-wheel zoom) is
Flutter's proven logic verbatim.

Also add an on-device input diagnostic (bug-report toggle): the existing pen
readout already prints kind/pressure/buttons; now it also shows live
zoom=now/min/max so the next device test captures (a) whether the side/eraser
button arrives as buttons/invertedStylus, and (b) the value any residual pinch
flash jumps to. 66/66 tests, analyze clean, linux build green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/canvas/pen_editor_screen.dart

@@ -95,6 +95,13 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   String _penDebug = '';
   bool _showPenDebug = false;
 
+  /// Live zoom-scale diagnostic (only tracked while the overlay is on), so we
+  /// can capture the value the pinch flash jumps to. `_zoomMin/_zoomMax` record
+  /// the extremes seen since the overlay was last enabled.
+  double _zoomNow = 1.0;
+  double _zoomMin = double.infinity;
+  double _zoomMax = 0.0;
+
   // Tool state.
   CanvasTool _tool = CanvasTool.pen;
   Color _color = Colors.black;
@@ -119,11 +126,23 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     // Begin listening to the native Windows pen plugin (barrel/eraser/tilt).
     // No-op on platforms without the plugin (W3).
     PenInputService.instance.start();
+    _transform.addListener(_onTransformDebug);
     _initPersistence();
     _initPenConfig();
     _open();
   }
 
+  /// Track the live zoom scale for the diagnostic overlay (no-op when off).
+  void _onTransformDebug() {
+    if (!_showPenDebug) return;
+    final s = _transform.value.getMaxScaleOnAxis();
+    setState(() {
+      _zoomNow = s;
+      if (s < _zoomMin) _zoomMin = s;
+      if (s > _zoomMax) _zoomMax = s;
+    });
+  }
+
   Future<void> _initPenConfig() async {
     final controller = await PenConfigController.load();
     if (!mounted) {
@@ -221,6 +240,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
       scheduler.dispose();
     }
     _document?.dispose();
+    _transform.removeListener(_onTransformDebug);
     _transform.dispose();
     _penConfig?.dispose();
     PenInputService.instance.stop();
@@ -437,9 +457,10 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                       padding: const EdgeInsets.symmetric(
                           horizontal: 10, vertical: 6),
                       child: Text(
-                        _penDebug.isEmpty
-                            ? 'hover / draw with the pen…'
-                            : _penDebug,
+                        '${_penDebug.isEmpty ? 'hover / draw with the pen…' : _penDebug}'
+                        '\nzoom=${_zoomNow.toStringAsFixed(2)}  '
+                        'min=${_zoomMin.isFinite ? _zoomMin.toStringAsFixed(2) : '-'}  '
+                        'max=${_zoomMax.toStringAsFixed(2)}',
                         style: TextStyle(
                           fontFamily: 'monospace',
                           fontSize: 12,
@@ -600,8 +621,15 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
             _ToolButton(
               icon: Icons.bug_report_outlined,
               selected: _showPenDebug,
-              tooltip: 'Pen pressure diagnostic',
-              onPressed: () => setState(() => _showPenDebug = !_showPenDebug),
+              tooltip: 'Pen + zoom diagnostic',
+              onPressed: () => setState(() {
+                _showPenDebug = !_showPenDebug;
+                if (_showPenDebug) {
+                  _zoomMin = double.infinity;
+                  _zoomMax = 0.0;
+                  _zoomNow = _transform.value.getMaxScaleOnAxis();
+                }
+              }),
             ),
           ],
         ),