feat(pen): persist brush kind so it survives reload

Closes TODO(brush-persist). EditorStroke now serializes its brush as
the stable BrushKind name; sidecars written before this field, and any
unknown name, load as fountainPen (back-compat). PenStroke<->EditorStroke
carry brush both ways, so a ballpoint/highlighter/pencil stroke keeps
its opacity/blend after a document is closed and reopened.

Note: InkStroke (the note/scratchpad world-coord format) has no brush
field, so notes derive brush from the tool — highlighter is preserved,
ballpoint/pencil collapse to fountainPen on reload (TODO: extend
InkStroke). PDF documents persist brush fully. analyze clean,
379 tests green.

test/badnote_sidecar_test.dart

@@ -10,6 +10,7 @@ import 'dart:convert';
 
 import 'package:flutter_test/flutter_test.dart';
 
+import 'package:badnote/editor/engine/brush.dart';
 import 'package:badnote/editor/engine/stroke_model.dart';
 import 'package:badnote/models/bookmark.dart';
 import 'package:badnote/models/ink_point.dart';
@@ -141,6 +142,43 @@ void main() {
     expect(sp.strokes, original.scratchLinks.single.scratchpad.strokes);
   });
 
+  test('sidecar with mixed-brush strokes round-trips each brush', () {
+    EditorStroke brushStroke(String id, BrushKind brush) => EditorStroke(
+          id: id,
+          points: const [
+            EditorPoint(x: 0.1, y: 0.2, pressure: 0.6),
+            EditorPoint(x: 0.4, y: 0.5, pressure: 0.6),
+          ],
+          color: 0xFF223344,
+          width: 0.004,
+          brush: brush,
+        );
+
+    final original = BadnoteSidecar(strokes: {
+      0: [
+        brushStroke('b-fountain', BrushKind.fountainPen),
+        brushStroke('b-ballpoint', BrushKind.ballpoint),
+      ],
+      1: [
+        brushStroke('b-pencil', BrushKind.pencil),
+        brushStroke('b-highlighter', BrushKind.highlighter),
+      ],
+    });
+    final reparsed = _roundTrip(original);
+
+    expect(
+      reparsed.strokes[0]!.map((s) => s.brush).toList(),
+      [BrushKind.fountainPen, BrushKind.ballpoint],
+    );
+    expect(
+      reparsed.strokes[1]!.map((s) => s.brush).toList(),
+      [BrushKind.pencil, BrushKind.highlighter],
+    );
+    // Full value equality (brush is part of EditorStroke's freezed equality).
+    expect(reparsed.strokes[0], original.strokes[0]);
+    expect(reparsed.strokes[1], original.strokes[1]);
+  });
+
   test('strokes JSON is byte-compatible with EditorStroke.toJson', () {
     final stroke = _editorStroke('s0', EditorTool.pen);
     final sidecar = BadnoteSidecar(strokes: {2: [stroke]});