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.

lib/editor/engine/stroke_model.g.dart

@@ -52,6 +52,13 @@ _$EditorStrokeImpl _$$EditorStrokeImplFromJson(Map<String, dynamic> json) =>
       filled: json['filled'] as bool? ?? false,
       textContent: json['textContent'] as String?,
       fontSize: (json['fontSize'] as num?)?.toDouble() ?? 14.0,
+      brush:
+          $enumDecodeNullable(
+            _$BrushKindEnumMap,
+            json['brush'],
+            unknownValue: BrushKind.fountainPen,
+          ) ??
+          BrushKind.fountainPen,
     );
 
 Map<String, dynamic> _$$EditorStrokeImplToJson(_$EditorStrokeImpl instance) =>
@@ -64,6 +71,7 @@ Map<String, dynamic> _$$EditorStrokeImplToJson(_$EditorStrokeImpl instance) =>
       'filled': instance.filled,
       'textContent': instance.textContent,
       'fontSize': instance.fontSize,
+      'brush': _$BrushKindEnumMap[instance.brush]!,
     };
 
 const _$EditorToolEnumMap = {
@@ -71,3 +79,10 @@ const _$EditorToolEnumMap = {
   EditorTool.highlighter: 'highlighter',
   EditorTool.eraser: 'eraser',
 };
+
+const _$BrushKindEnumMap = {
+  BrushKind.fountainPen: 'fountainPen',
+  BrushKind.ballpoint: 'ballpoint',
+  BrushKind.highlighter: 'highlighter',
+  BrushKind.pencil: 'pencil',
+};