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.dart

@@ -13,6 +13,12 @@
 //   * Point x/y are NORMALIZED to the page rectangle, i.e. in [0,1].
 //   * Stroke `width` is a FRACTION of the page width, so it scales with zoom.
 
+// @JsonKey is applied to freezed factory parameters (e.g. EditorStroke.brush)
+// for fine-grained serialization control; freezed re-emits those annotations on
+// generated getters where they're valid, so suppress the source-level
+// invalid_annotation_target for the whole file (the documented freezed pattern).
+// ignore_for_file: invalid_annotation_target
+
 import 'package:freezed_annotation/freezed_annotation.dart';
 import 'package:uuid/uuid.dart';
 
@@ -75,13 +81,17 @@ abstract class EditorStroke with _$EditorStroke {
     @Default(false) bool filled,
     String? textContent,
     @Default(14.0) double fontSize,
-    // Brush the stroke was drawn with. NOT serialized (increment 1 keeps brush
-    // out of persistence / InkStroke round-trip — see TODO(brush-persist)); it
-    // is an IN-MEMORY render hint only, so the committed render path can resolve
-    // each stroke's perfect_freehand geometry. Defaults to fountainPen so loaded
-    // (deserialized) strokes keep the legacy pen visual.
-    // ignore: invalid_annotation_target
-    @JsonKey(includeFromJson: false, includeToJson: false)
+    // Brush the stroke was drawn with — drives the committed render path's
+    // perfect_freehand geometry + opacity/blend (resolveStrokePaint). Persisted
+    // as the stable `BrushKind` @JsonValue name (e.g. "ballpoint") so a
+    // ballpoint/highlighter/pencil stroke keeps its look across close/reopen.
+    // BACK-COMPAT: sidecars written before this field existed have no `brush`
+    // key, and an unknown name (a future brush opened by an older build) is
+    // tolerated — both fall back to fountainPen via the JsonKey below.
+    @JsonKey(
+      defaultValue: BrushKind.fountainPen,
+      unknownEnumValue: BrushKind.fountainPen,
+    )
     @Default(BrushKind.fountainPen)
     BrushKind brush,
   }) = _EditorStroke;
@@ -154,6 +164,11 @@ abstract class EditorStroke with _$EditorStroke {
         filled: stroke.filled,
         textContent: stroke.textContent,
         fontSize: stroke.fontSize,
+        // InkStroke has no brush field; derive from the tool so a loaded
+        // highlighter keeps the flat highlighter brush (pens → fountainPen).
+        brush: stroke.tool == PenTool.highlighter
+            ? BrushKind.highlighter
+            : BrushKind.fountainPen,
       );
 
   /// Lossless adapter to the freezed/JSON [InkStroke] model. Null superset