feat(pen): extensible brush model (4 brushes)

Replace the 2-tool ink system with a data-driven, Krita-style
BrushProfile (lib/editor/engine/brush.dart). Adding a brush is a
const map entry, not render-path branching.

Four presets from the rnote/krita spec:
- fountain pen: quadratic (p^2) pressure, wide dynamic width
- ballpoint:    near-constant width (thinning 0.15)
- highlighter:  flat width, square caps
- pencil:       sqrt(p) pressure, moderate width

Pressure is pre-warped per brush via PressureCurve(gamma) before
perfect_freehand; geometry fields (thinning/streamline/smoothing/
caps) flow through the shared stroke recipe so the PDF overlay and
the note/slide PenCanvas both honor the brush. Brush kind is now
persisted on the stroke model. Picker added to all three toolbars.

Opacity/multiply and pencil grain are carried as data but not yet
composited (TODO brush-opacity / brush-texture); this increment is
width + pressure-curve differentiation. analyze clean, 283 tests.

lib/editor/engine/stroke_model.dart

@@ -21,6 +21,7 @@ import '../../models/ink_stroke.dart';
 import '../../models/pen_tool.dart';
 import '../../models/pointer_device_kind.dart';
 import '../canvas/pen_stroke.dart';
+import 'brush.dart';
 
 part 'stroke_model.freezed.dart';
 part 'stroke_model.g.dart';
@@ -74,6 +75,15 @@ 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)
+    @Default(BrushKind.fountainPen)
+    BrushKind brush,
   }) = _EditorStroke;
 
   /// Convenience constructor that generates a uuid [id] when none is supplied.
@@ -86,6 +96,7 @@ abstract class EditorStroke with _$EditorStroke {
     bool filled = false,
     String? textContent,
     double fontSize = 14.0,
+    BrushKind brush = BrushKind.fountainPen,
   }) =>
       EditorStroke(
         id: id ?? _uuid.v4(),
@@ -96,6 +107,7 @@ abstract class EditorStroke with _$EditorStroke {
         filled: filled,
         textContent: textContent,
         fontSize: fontSize,
+        brush: brush,
       );
 
   factory EditorStroke.fromJson(Map<String, dynamic> json) =>
@@ -118,6 +130,7 @@ abstract class EditorStroke with _$EditorStroke {
         },
         color: stroke.color,
         width: stroke.width,
+        brush: stroke.brush,
       );
 
   /// Lossless adapter from the freezed/JSON [InkStroke] model.