fix(pen): eraser lag/stuck-red/reliability; zoom glitch-reject; native input diag

Eraser (regression from the preview I added):
- LAG: the preview did setState on every hover/erase-move (rebuilding the whole
  canvas) and recomputed perfect_freehand getStroke per overlapped stroke per
  frame. Now the cursor is a ValueNotifier driving the preview layer's repaint
  directly (no canvas rebuild), and the highlight is a plain polyline of the
  point-runs inside the radius (no getStroke).
- STUCK RED ("一直红着"): the cursor was never cleared. Preview is now
  active-erase-only and cleared on pen up/cancel.
- "选中了的笔画也不见得能删掉": radius was strokeWidth*2 (tiny) so a pass removed
  ~2 points and the stroke survived. Now a decisive fixed 0.02 (page-width
  fraction). The highlight traces exactly the point-run that splitStrokeByCircle
  removes, so what turns red is what gets deleted.

Zoom: replace the per-frame scale CLAMP with glitch REJECTION — drop a frame
demanding an implausible per-frame scale jump (>1.4x or <0.71x; a real pinch is
≲1.15x/frame). A dropped frame catches up the next frame (absolute tracking), so
no lag, but the Windows multi-touch spike never shows. Pairs with the existing
pointer-count re-baseline.

Native diagnostic: ObservePenMessage now counts WM_POINTER* / PT_PEN / legacy
mouse messages it sees and emits them on the channel; PenInputService exposes
`debugSummary` and the overlay shows `native ptr=… pen=… mouse=… msg=0x…`. This
will tell us on-device whether WM_POINTER ever reaches the observer (→ buttons
recoverable) or Flutter is on a non-pointer path (→ not).

Dart: analyze clean, 66/66 tests, linux build green. Native compiles on CI.

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

lib/editor/canvas/pen_canvas.dart

@@ -130,8 +130,10 @@ class _PenCanvasState extends State<PenCanvas> {
   bool _eraserActive = false;
 
   /// Eraser preview cursor (normalized page coords), or null when not in eraser
-  /// mode / the pen is not near the page. Drives [EraserPreviewPainter].
-  PenPoint? _eraserCursor;
+  /// mode / the pen is not near the page. A ValueNotifier so the preview layer
+  /// repaints on cursor moves WITHOUT rebuilding the whole canvas every frame
+  /// (the old per-move setState was the eraser-lag source).
+  final ValueNotifier<PenPoint?> _eraserCursor = ValueNotifier<PenPoint?>(null);
 
   /// True when the eraser would act (eraser tool selected, or a barrel/inverted
   /// eraser signal is live).
@@ -139,23 +141,16 @@ class _PenCanvasState extends State<PenCanvas> {
       widget.tool == CanvasTool.eraser || _eraserActive;
 
   /// Eraser radius as a fraction of page width (shared by the live erase and the
-  /// preview overlay so they always agree).
-  double get _eraserRadius => widget.strokeWidth * 2;
+  /// preview overlay so they always agree). A decisive fixed size — the old
+  /// strokeWidth*2 was so small that a pass removed only a couple of points and
+  /// the stroke visibly survived ("选中了的笔画也不见得能删掉").
+  static const double _eraserRadius = 0.02;
 
   /// Page aspect (height / width) so the eraser circle stays round on screen.
   double get _pageAspect => widget.pageSize.width <= 0
       ? 1.0
       : widget.pageSize.height / widget.pageSize.width;
 
-  /// Update (or clear) the eraser-preview cursor from a global pointer position.
-  void _updateEraserCursor(Offset globalPosition) {
-    if (_isEraserMode) {
-      setState(() => _eraserCursor = _toNormalized(globalPosition, null));
-    } else if (_eraserCursor != null) {
-      setState(() => _eraserCursor = null);
-    }
-  }
-
   // The explicit user toggle wins: if finger-drawing is ON, a single finger
   // draws even after a stylus has been seen. (Palm rejection when the toggle is
   // OFF is automatic — fingers simply never draw — and a 2nd pointer always
@@ -295,12 +290,12 @@ class _PenCanvasState extends State<PenCanvas> {
     if (p != null) _livePoints.add(p);
 
     if (_eraserActive || widget.tool == CanvasTool.eraser) {
+      _eraserCursor.value = p;
       _eraseAt(p);
-      // Keep the stroke pointer reserved so moves keep erasing, but don't paint.
-      setState(() {
-        _liveStroke = null;
-        _eraserCursor = p;
-      });
+      // No setState here: the preview repaints via the notifier, and any erased
+      // stroke repaints via the editor's onEraseStroke setState. (_liveStroke is
+      // already null in eraser mode.)
+      if (_liveStroke != null) setState(() => _liveStroke = null);
       return;
     }
     _updateLiveStroke();
@@ -312,8 +307,8 @@ class _PenCanvasState extends State<PenCanvas> {
     if (p == null) return;
 
     if (_eraserActive || widget.tool == CanvasTool.eraser) {
+      _eraserCursor.value = p;
       _eraseAt(p);
-      setState(() => _eraserCursor = p);
       return;
     }
     _livePoints.add(p);
@@ -335,6 +330,7 @@ class _PenCanvasState extends State<PenCanvas> {
     }
     _drawPointer = null;
     _livePoints.clear();
+    _eraserCursor.value = null; // hide the preview when the pen lifts
     setState(() => _liveStroke = null);
   }
 
@@ -342,6 +338,7 @@ class _PenCanvasState extends State<PenCanvas> {
   void _cancelStroke() {
     _drawPointer = null;
     _livePoints.clear();
+    _eraserCursor.value = null;
     setState(() => _liveStroke = null);
   }
 
@@ -400,7 +397,8 @@ class _PenCanvasState extends State<PenCanvas> {
         '/${event.pressureMax.toStringAsFixed(0)}  '
         'norm=${norm?.toStringAsFixed(3) ?? "null"}  '
         'peak=${_peakNorm.toStringAsFixed(3)}  '
-        'btn=${event.buttons}  tilt=${event.tilt.toStringAsFixed(2)}');
+        'btn=${event.buttons}  tilt=${event.tilt.toStringAsFixed(2)}'
+        '\n${PenInputService.instance.debugSummary}');
   }
 
   void _onPointerHover(PointerHoverEvent event) {
@@ -411,8 +409,6 @@ class _PenCanvasState extends State<PenCanvas> {
       _dispatchHwButtonActions();
       // Detect eraser (barrel button / inverted) while hovering.
       _eraserActive = _isEraserSignal(event);
-      // Live eraser-preview cursor follows the hovering pen.
-      _updateEraserCursor(event.position);
     }
   }
 
@@ -467,6 +463,12 @@ class _PenCanvasState extends State<PenCanvas> {
     if (wasDrawer) _cancelStroke();
   }
 
+  @override
+  void dispose() {
+    _eraserCursor.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     // The PEN never reaches PenInteractiveViewer's recognizer (it excludes
@@ -516,7 +518,7 @@ class _PenCanvasState extends State<PenCanvas> {
               ),
               // Eraser preview: faint outline on strokes about to be deleted +
               // the eraser circle. Mounted only in eraser mode with a cursor.
-              if (_isEraserMode && _eraserCursor != null)
+              if (_isEraserMode)
                 Positioned.fill(
                   child: RepaintBoundary(
                     child: CustomPaint(
@@ -526,7 +528,6 @@ class _PenCanvasState extends State<PenCanvas> {
                         radius: _eraserRadius,
                         aspect: _pageAspect,
                         pageSize: widget.pageSize,
-                        thinning: widget.thinning,
                       ),
                     ),
                   ),