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/canvas/pen_editor_screen.dart

@@ -339,13 +339,11 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                 kind: es.tool == EditorTool.highlighter
                     ? PenStrokeKind.highlighter
                     : PenStrokeKind.pen,
-                // Brush isn't persisted yet (TODO(brush-persist)); derive it
-                // from the tool so a loaded highlighter still renders with the
-                // highlighter brush (flat width), and pens fall back to the
-                // fountainPen default.
-                brush: es.tool == EditorTool.highlighter
-                    ? BrushKind.highlighter
-                    : BrushKind.fountainPen,
+                // Brush is now persisted on the EditorStroke; carry it through
+                // so a reopened ballpoint/pencil/highlighter keeps its
+                // opacity/blend. Old sidecars without the field decode to
+                // fountainPen (see EditorStroke.brush back-compat default).
+                brush: es.brush,
               ))
           .toList();
     }