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/ink_painters.dart

@@ -4,10 +4,10 @@
 // coordinates; both painters receive the on-screen page [Size] and scale
 // points into pixels at paint time. perfect_freehand produces the outline.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:perfect_freehand/perfect_freehand.dart' as pf;
 
-import '../engine/stroke_eraser.dart' show strokeHit;
 import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import 'pen_stroke.dart';
 
@@ -116,13 +116,13 @@ class EraserPreviewPainter extends CustomPainter {
     required this.radius,
     required this.aspect,
     required this.pageSize,
-    this.thinning = kDefaultPenThinning,
-  });
+  }) : super(repaint: cursor);
 
   final List<PenStroke> strokes;
 
-  /// Eraser center in normalized page coords, or null when no preview.
-  final PenPoint? cursor;
+  /// Eraser center in normalized page coords (null = no preview). A listenable
+  /// so the painter repaints on cursor moves WITHOUT rebuilding the canvas.
+  final ValueListenable<PenPoint?> cursor;
 
   /// Eraser radius as a fraction of page width (matches the live erase test).
   final double radius;
@@ -131,25 +131,44 @@ class EraserPreviewPainter extends CustomPainter {
   final double aspect;
 
   final Size pageSize;
-  final double thinning;
 
   @override
   void paint(Canvas canvas, Size size) {
-    final c = cursor;
+    final c = cursor.value;
     if (c == null) return;
 
-    // Faint outline on each stroke the eraser currently overlaps.
+    // CHEAP, ACCURATE highlight: trace ONLY the point-runs inside the eraser
+    // radius — i.e. exactly what splitStrokeByCircle will remove — as a plain
+    // polyline (no perfect_freehand getStroke; that was the eraser lag source).
+    // So what turns red is exactly what gets deleted.
+    final r2 = radius * radius;
     final highlight = Paint()
-      ..color = const Color(0xFFFF5252).withValues(alpha: 0.55)
+      ..color = const Color(0xFFFF5252).withValues(alpha: 0.85)
       ..style = PaintingStyle.stroke
-      ..strokeWidth = 1.5
+      ..strokeWidth = 3.0
+      ..strokeCap = StrokeCap.round
+      ..strokeJoin = StrokeJoin.round
       ..isAntiAlias = true;
     for (final stroke in strokes) {
-      if (!strokeHit(stroke, c.x, c.y, radius, aspect: aspect)) continue;
-      final path =
-          buildStrokePath(stroke, pageSize, isComplete: true, thinning: thinning);
-      if (path.getBounds().isEmpty) continue;
-      canvas.drawPath(path, highlight);
+      Path? run;
+      void flush() {
+        if (run != null) {
+          canvas.drawPath(run!, highlight);
+          run = null;
+        }
+      }
+
+      for (final pt in stroke.points) {
+        final dx = pt.x - c.x;
+        final dy = (pt.y - c.y) * aspect;
+        if (dx * dx + dy * dy < r2) {
+          final o = Offset(pt.x * pageSize.width, pt.y * pageSize.height);
+          (run ??= Path()..moveTo(o.dx, o.dy)).lineTo(o.dx, o.dy);
+        } else {
+          flush();
+        }
+      }
+      flush();
     }
 
     // The eraser circle itself (radius is a page-width fraction → px = r * w).
@@ -159,7 +178,7 @@ class EraserPreviewPainter extends CustomPainter {
       center,
       rPx,
       Paint()
-        ..color = const Color(0xFF9E9E9E).withValues(alpha: 0.5)
+        ..color = const Color(0xFF757575).withValues(alpha: 0.7)
         ..style = PaintingStyle.stroke
         ..strokeWidth = 1.0
         ..isAntiAlias = true,
@@ -167,22 +186,18 @@ class EraserPreviewPainter extends CustomPainter {
     canvas.drawCircle(
       center,
       rPx,
-      Paint()
-        ..color = const Color(0x14000000)
-        ..style = PaintingStyle.fill,
+      Paint()..color = const Color(0x14000000),
     );
   }
 
   @override
   bool shouldRepaint(EraserPreviewPainter old) =>
-      old.cursor?.x != cursor?.x ||
-      old.cursor?.y != cursor?.y ||
+      !identical(old.cursor, cursor) ||
       old.radius != radius ||
       old.aspect != aspect ||
       !identical(old.strokes, strokes) ||
       old.strokes.length != strokes.length ||
-      old.pageSize != pageSize ||
-      old.thinning != thinning;
+      old.pageSize != pageSize;
 }
 
 /// Paints just the in-progress stroke (the live layer), kept behind its own