feat: stroke spatial bounds + broad-phase visibility (board culling)

strokeBounds (tight AABB over normalized points, null for empty, zero-size for a
single point), strokesBounds (union), and strokeIntersects (does a stroke's box
overlap a viewport rect — touching edges count). Broad-phase primitive for the
infinite board: skip painting/erasing/hit-testing strokes off-screen (R1 perf),
and a cheap pre-filter before the exact per-point eraser test.

Pure geometry over EditorStroke; fully unit-tested.

flutter analyze lib/editor clean; 191/191 tests (+10).

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

test/stroke_bounds_test.dart

@@ -0,0 +1,97 @@
+// Tests for stroke spatial bounds + broad-phase visibility (F7 board culling).
+
+import 'dart:ui' show Rect;
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/engine/stroke_model.dart';
+import 'package:badnote/editor/engine/stroke_bounds.dart';
+
+EditorStroke _stroke(List<List<double>> pts) => EditorStroke.create(
+      id: 's',
+      points: [for (final p in pts) EditorPoint(x: p[0], y: p[1])],
+    );
+
+void main() {
+  test('strokeBounds is the tight box over points', () {
+    final b = strokeBounds(_stroke([
+      [0.1, 0.2],
+      [0.5, 0.1],
+      [0.3, 0.6],
+    ]))!;
+    expect(b.left, closeTo(0.1, 1e-9));
+    expect(b.top, closeTo(0.1, 1e-9));
+    expect(b.right, closeTo(0.5, 1e-9));
+    expect(b.bottom, closeTo(0.6, 1e-9));
+  });
+
+  test('empty stroke → null', () {
+    expect(strokeBounds(_stroke([])), isNull);
+  });
+
+  test('single point → zero-size rect at that point', () {
+    final b = strokeBounds(_stroke([
+      [0.4, 0.7],
+    ]))!;
+    expect(b.width, 0);
+    expect(b.height, 0);
+    expect(b.left, closeTo(0.4, 1e-9));
+    expect(b.top, closeTo(0.7, 1e-9));
+  });
+
+  test('strokesBounds is the union; skips empty strokes', () {
+    final b = strokesBounds([
+      _stroke([
+        [0.1, 0.1],
+        [0.2, 0.2],
+      ]),
+      _stroke([]), // skipped
+      _stroke([
+        [0.8, 0.7],
+      ]),
+    ])!;
+    expect(b.left, closeTo(0.1, 1e-9));
+    expect(b.top, closeTo(0.1, 1e-9));
+    expect(b.right, closeTo(0.8, 1e-9));
+    expect(b.bottom, closeTo(0.7, 1e-9));
+  });
+
+  test('strokesBounds of all-empty → null', () {
+    expect(strokesBounds([_stroke([]), _stroke([])]), isNull);
+  });
+
+  group('strokeIntersects (broad-phase)', () {
+    final view = const Rect.fromLTRB(0.0, 0.0, 0.5, 0.5);
+
+    test('inside the viewport intersects', () {
+      expect(strokeIntersects(_stroke([
+            [0.1, 0.1],
+            [0.2, 0.2],
+          ]), view), isTrue);
+    });
+
+    test('fully outside does NOT intersect', () {
+      expect(strokeIntersects(_stroke([
+            [0.8, 0.8],
+            [0.9, 0.9],
+          ]), view), isFalse);
+    });
+
+    test('partial overlap intersects', () {
+      expect(strokeIntersects(_stroke([
+            [0.4, 0.4],
+            [0.7, 0.7],
+          ]), view), isTrue);
+    });
+
+    test('touching edge counts as intersecting', () {
+      expect(strokeIntersects(_stroke([
+            [0.5, 0.5],
+          ]), view), isTrue);
+    });
+
+    test('empty stroke is never visible', () {
+      expect(strokeIntersects(_stroke([]), view), isFalse);
+    });
+  });
+}