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>

lib/editor/engine/stroke_bounds.dart

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+// lib/editor/engine/stroke_bounds.dart
+//
+// Axis-aligned bounds of strokes in normalized content coordinates. Used for
+// broad-phase culling (don't paint/erase/hit-test strokes whose box is off the
+// viewport — the infinite board's R1 perf primitive), and as a cheap pre-filter
+// before the exact per-point eraser test.
+//
+// Pure geometry over EditorStroke; no widgets/storage; fully unit-tested.
+
+import 'dart:ui' show Rect;
+
+import 'stroke_model.dart';
+
+/// Tight axis-aligned bounds of [stroke] in normalized coords, or null when the
+/// stroke has no points. A single-point stroke yields a zero-size rect at that
+/// point.
+Rect? strokeBounds(EditorStroke stroke) {
+  if (stroke.points.isEmpty) return null;
+  var minX = double.infinity, minY = double.infinity;
+  var maxX = double.negativeInfinity, maxY = double.negativeInfinity;
+  for (final p in stroke.points) {
+    if (p.x < minX) minX = p.x;
+    if (p.y < minY) minY = p.y;
+    if (p.x > maxX) maxX = p.x;
+    if (p.y > maxY) maxY = p.y;
+  }
+  return Rect.fromLTRB(minX, minY, maxX, maxY);
+}
+
+/// Union bounds of [strokes], or null when none have points.
+Rect? strokesBounds(Iterable<EditorStroke> strokes) {
+  Rect? acc;
+  for (final stroke in strokes) {
+    final b = strokeBounds(stroke);
+    if (b == null) continue;
+    acc = acc == null ? b : acc.expandToInclude(b);
+  }
+  return acc;
+}
+
+/// Whether [stroke]'s bounds overlap [viewport] (broad-phase visibility test).
+/// Empty strokes are never visible. Touching edges count as overlapping.
+bool strokeIntersects(EditorStroke stroke, Rect viewport) {
+  final b = strokeBounds(stroke);
+  if (b == null) return false;
+  return b.left <= viewport.right &&
+      b.right >= viewport.left &&
+      b.top <= viewport.bottom &&
+      b.bottom >= viewport.top;
+}