test: ink coordinate mapping through zoom (device-independent)

A stylus point drawn at 2x zoom maps to the correct normalized page coordinate
(canvas-local 200,300 → scene 100,150 → normalized 0.25,0.25), guarding that ink
stays glued to the page under zoom/pan — the core own-canvas invariant.

flutter analyze lib/editor clean; 238/238 tests (+1).

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

test/pen_coordinate_widget_test.dart

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+// Device-independent correctness guard: a drawn point must map to the right
+// NORMALIZED page coordinate through a non-identity (zoomed) transform, so ink
+// stays glued to the page under zoom/pan.
+
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/canvas/pen_canvas.dart';
+import 'package:badnote/editor/canvas/pen_stroke.dart';
+
+void main() {
+  const pageSize = Size(400, 600);
+
+  testWidgets('a stylus point maps to correct normalized coords at 2x zoom',
+      (tester) async {
+    final committed = <PenStroke>[];
+    final controller = TransformationController()
+      ..value = (Matrix4.identity()..scaleByDouble(2.0, 2.0, 2.0, 1));
+    addTearDown(controller.dispose);
+
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Center(
+          child: SizedBox(
+            width: pageSize.width,
+            height: pageSize.height,
+            child: PenCanvas(
+              pageWidget: Container(color: const Color(0xFFEEEEEE)),
+              pageSize: pageSize,
+              strokes: const <PenStroke>[],
+              transformationController: controller,
+              tool: CanvasTool.pen,
+              color: const Color(0xFF000000),
+              strokeWidth: 0.004,
+              onStrokeComplete: committed.add,
+              onEraseStroke: (_, __) {},
+            ),
+          ),
+        ),
+      ),
+    ));
+
+    // Down at the canvas-local point (200, 300). At 2x scale, scene = (100,150),
+    // normalized = (100/400, 150/600) = (0.25, 0.25).
+    final topLeft = tester.getTopLeft(find.byType(PenCanvas));
+    final g = await tester.startGesture(topLeft + const Offset(200, 300),
+        kind: PointerDeviceKind.stylus);
+    await g.moveBy(const Offset(8, 8)); // make it a stroke, not a tap
+    await g.up();
+    await tester.pump();
+
+    expect(committed, hasLength(1));
+    final first = committed.single.points.first;
+    expect(first.x, closeTo(0.25, 1e-3));
+    expect(first.y, closeTo(0.25, 1e-3));
+  });
+}