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/brush.dart

@@ -20,19 +20,31 @@
 
 import 'dart:ui' show Color, BlendMode;
 
+import 'package:freezed_annotation/freezed_annotation.dart';
+
 /// The four selectable brushes. Extensible: add a kind here + a preset in
 /// [kBrushPresets]. The eraser is NOT a brush — it stays a separate tool.
+///
+/// The `@JsonValue` names are the STABLE on-disk identifiers persisted in the
+/// sidecar (`EditorStroke.brush`); they are decoupled from the Dart enum
+/// identifiers so renaming a constant here never breaks existing sidecars. A
+/// brush whose stored name is unknown (e.g. a future brush opened by an older
+/// build) is read back as [fountainPen] (see `EditorStroke.brush`'s JsonKey).
 enum BrushKind {
   /// Strong pressure→width (rnote Pow2 / quadratic), soft taper, solid ink.
+  @JsonValue('fountainPen')
   fountainPen,
 
   /// Near-constant thin width; pressure carries OPACITY (the ballpoint "tell").
+  @JsonValue('ballpoint')
   ballpoint,
 
   /// Broad, flat width, translucent, square (uncapped) ends.
+  @JsonValue('highlighter')
   highlighter,
 
   /// Moderate width + opacity from pressure (rnote Sqrt / √p), scratchy.
+  @JsonValue('pencil')
   pencil,
 }