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.freezed.dart

@@ -303,12 +303,17 @@ mixin _$EditorStroke {
   bool get filled => throw _privateConstructorUsedError;
   String? get textContent => throw _privateConstructorUsedError;
   double get fontSize =>
-      throw _privateConstructorUsedError; // Brush the stroke was drawn with. NOT serialized (increment 1 keeps brush
-  // out of persistence / InkStroke round-trip — see TODO(brush-persist)); it
-  // is an IN-MEMORY render hint only, so the committed render path can resolve
-  // each stroke's perfect_freehand geometry. Defaults to fountainPen so loaded
-  // (deserialized) strokes keep the legacy pen visual.
-  @JsonKey(includeFromJson: false, includeToJson: false)
+      throw _privateConstructorUsedError; // Brush the stroke was drawn with — drives the committed render path's
+  // perfect_freehand geometry + opacity/blend (resolveStrokePaint). Persisted
+  // as the stable `BrushKind` @JsonValue name (e.g. "ballpoint") so a
+  // ballpoint/highlighter/pencil stroke keeps its look across close/reopen.
+  // BACK-COMPAT: sidecars written before this field existed have no `brush`
+  // key, and an unknown name (a future brush opened by an older build) is
+  // tolerated — both fall back to fountainPen via the JsonKey below.
+  @JsonKey(
+    defaultValue: BrushKind.fountainPen,
+    unknownEnumValue: BrushKind.fountainPen,
+  )
   BrushKind get brush => throw _privateConstructorUsedError;
 
   /// Serializes this EditorStroke to a JSON map.
@@ -337,7 +342,11 @@ abstract class $EditorStrokeCopyWith<$Res> {
     bool filled,
     String? textContent,
     double fontSize,
-    @JsonKey(includeFromJson: false, includeToJson: false) BrushKind brush,
+    @JsonKey(
+      defaultValue: BrushKind.fountainPen,
+      unknownEnumValue: BrushKind.fountainPen,
+    )
+    BrushKind brush,
   });
 }
 
@@ -428,7 +437,11 @@ abstract class _$$EditorStrokeImplCopyWith<$Res>
     bool filled,
     String? textContent,
     double fontSize,
-    @JsonKey(includeFromJson: false, includeToJson: false) BrushKind brush,
+    @JsonKey(
+      defaultValue: BrushKind.fountainPen,
+      unknownEnumValue: BrushKind.fountainPen,
+    )
+    BrushKind brush,
   });
 }
 
@@ -511,7 +524,10 @@ class _$EditorStrokeImpl extends _EditorStroke {
     this.filled = false,
     this.textContent,
     this.fontSize = 14.0,
-    @JsonKey(includeFromJson: false, includeToJson: false)
+    @JsonKey(
+      defaultValue: BrushKind.fountainPen,
+      unknownEnumValue: BrushKind.fountainPen,
+    )
     this.brush = BrushKind.fountainPen,
   }) : _points = points,
        super._();
@@ -546,13 +562,18 @@ class _$EditorStrokeImpl extends _EditorStroke {
   @override
   @JsonKey()
   final double fontSize;
-  // Brush the stroke was drawn with. NOT serialized (increment 1 keeps brush
-  // out of persistence / InkStroke round-trip — see TODO(brush-persist)); it
-  // is an IN-MEMORY render hint only, so the committed render path can resolve
-  // each stroke's perfect_freehand geometry. Defaults to fountainPen so loaded
-  // (deserialized) strokes keep the legacy pen visual.
+  // Brush the stroke was drawn with — drives the committed render path's
+  // perfect_freehand geometry + opacity/blend (resolveStrokePaint). Persisted
+  // as the stable `BrushKind` @JsonValue name (e.g. "ballpoint") so a
+  // ballpoint/highlighter/pencil stroke keeps its look across close/reopen.
+  // BACK-COMPAT: sidecars written before this field existed have no `brush`
+  // key, and an unknown name (a future brush opened by an older build) is
+  // tolerated — both fall back to fountainPen via the JsonKey below.
   @override
-  @JsonKey(includeFromJson: false, includeToJson: false)
+  @JsonKey(
+    defaultValue: BrushKind.fountainPen,
+    unknownEnumValue: BrushKind.fountainPen,
+  )
   final BrushKind brush;
 
   @override
@@ -617,7 +638,10 @@ abstract class _EditorStroke extends EditorStroke {
     final bool filled,
     final String? textContent,
     final double fontSize,
-    @JsonKey(includeFromJson: false, includeToJson: false)
+    @JsonKey(
+      defaultValue: BrushKind.fountainPen,
+      unknownEnumValue: BrushKind.fountainPen,
+    )
     final BrushKind brush,
   }) = _$EditorStrokeImpl;
   _EditorStroke._() : super._();
@@ -640,13 +664,18 @@ abstract class _EditorStroke extends EditorStroke {
   @override
   String? get textContent;
   @override
-  double get fontSize; // Brush the stroke was drawn with. NOT serialized (increment 1 keeps brush
-  // out of persistence / InkStroke round-trip — see TODO(brush-persist)); it
-  // is an IN-MEMORY render hint only, so the committed render path can resolve
-  // each stroke's perfect_freehand geometry. Defaults to fountainPen so loaded
-  // (deserialized) strokes keep the legacy pen visual.
+  double get fontSize; // Brush the stroke was drawn with — drives the committed render path's
+  // perfect_freehand geometry + opacity/blend (resolveStrokePaint). Persisted
+  // as the stable `BrushKind` @JsonValue name (e.g. "ballpoint") so a
+  // ballpoint/highlighter/pencil stroke keeps its look across close/reopen.
+  // BACK-COMPAT: sidecars written before this field existed have no `brush`
+  // key, and an unknown name (a future brush opened by an older build) is
+  // tolerated — both fall back to fountainPen via the JsonKey below.
   @override
-  @JsonKey(includeFromJson: false, includeToJson: false)
+  @JsonKey(
+    defaultValue: BrushKind.fountainPen,
+    unknownEnumValue: BrushKind.fountainPen,
+  )
   BrushKind get brush;
 
   /// Create a copy of EditorStroke