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.freezed.dart

@@ -302,7 +302,14 @@ mixin _$EditorStroke {
   double get width => throw _privateConstructorUsedError;
   bool get filled => throw _privateConstructorUsedError;
   String? get textContent => throw _privateConstructorUsedError;
-  double get fontSize => 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)
+  BrushKind get brush => throw _privateConstructorUsedError;
 
   /// Serializes this EditorStroke to a JSON map.
   Map<String, dynamic> toJson() => throw _privateConstructorUsedError;
@@ -330,6 +337,7 @@ abstract class $EditorStrokeCopyWith<$Res> {
     bool filled,
     String? textContent,
     double fontSize,
+    @JsonKey(includeFromJson: false, includeToJson: false) BrushKind brush,
   });
 }
 
@@ -356,6 +364,7 @@ class _$EditorStrokeCopyWithImpl<$Res, $Val extends EditorStroke>
     Object? filled = null,
     Object? textContent = freezed,
     Object? fontSize = null,
+    Object? brush = null,
   }) {
     return _then(
       _value.copyWith(
@@ -391,6 +400,10 @@ class _$EditorStrokeCopyWithImpl<$Res, $Val extends EditorStroke>
                 ? _value.fontSize
                 : fontSize // ignore: cast_nullable_to_non_nullable
                       as double,
+            brush: null == brush
+                ? _value.brush
+                : brush // ignore: cast_nullable_to_non_nullable
+                      as BrushKind,
           )
           as $Val,
     );
@@ -415,6 +428,7 @@ abstract class _$$EditorStrokeImplCopyWith<$Res>
     bool filled,
     String? textContent,
     double fontSize,
+    @JsonKey(includeFromJson: false, includeToJson: false) BrushKind brush,
   });
 }
 
@@ -440,6 +454,7 @@ class __$$EditorStrokeImplCopyWithImpl<$Res>
     Object? filled = null,
     Object? textContent = freezed,
     Object? fontSize = null,
+    Object? brush = null,
   }) {
     return _then(
       _$EditorStrokeImpl(
@@ -475,6 +490,10 @@ class __$$EditorStrokeImplCopyWithImpl<$Res>
             ? _value.fontSize
             : fontSize // ignore: cast_nullable_to_non_nullable
                   as double,
+        brush: null == brush
+            ? _value.brush
+            : brush // ignore: cast_nullable_to_non_nullable
+                  as BrushKind,
       ),
     );
   }
@@ -492,6 +511,8 @@ class _$EditorStrokeImpl extends _EditorStroke {
     this.filled = false,
     this.textContent,
     this.fontSize = 14.0,
+    @JsonKey(includeFromJson: false, includeToJson: false)
+    this.brush = BrushKind.fountainPen,
   }) : _points = points,
        super._();
 
@@ -525,10 +546,18 @@ 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.
+  @override
+  @JsonKey(includeFromJson: false, includeToJson: false)
+  final BrushKind brush;
 
   @override
   String toString() {
-    return 'EditorStroke(id: $id, points: $points, tool: $tool, color: $color, width: $width, filled: $filled, textContent: $textContent, fontSize: $fontSize)';
+    return 'EditorStroke(id: $id, points: $points, tool: $tool, color: $color, width: $width, filled: $filled, textContent: $textContent, fontSize: $fontSize, brush: $brush)';
   }
 
   @override
@@ -545,7 +574,8 @@ class _$EditorStrokeImpl extends _EditorStroke {
             (identical(other.textContent, textContent) ||
                 other.textContent == textContent) &&
             (identical(other.fontSize, fontSize) ||
-                other.fontSize == fontSize));
+                other.fontSize == fontSize) &&
+            (identical(other.brush, brush) || other.brush == brush));
   }
 
   @JsonKey(includeFromJson: false, includeToJson: false)
@@ -560,6 +590,7 @@ class _$EditorStrokeImpl extends _EditorStroke {
     filled,
     textContent,
     fontSize,
+    brush,
   );
 
   /// Create a copy of EditorStroke
@@ -586,6 +617,8 @@ abstract class _EditorStroke extends EditorStroke {
     final bool filled,
     final String? textContent,
     final double fontSize,
+    @JsonKey(includeFromJson: false, includeToJson: false)
+    final BrushKind brush,
   }) = _$EditorStrokeImpl;
   _EditorStroke._() : super._();
 
@@ -607,7 +640,14 @@ abstract class _EditorStroke extends EditorStroke {
   @override
   String? get textContent;
   @override
-  double get fontSize;
+  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.
+  @override
+  @JsonKey(includeFromJson: false, includeToJson: false)
+  BrushKind get brush;
 
   /// Create a copy of EditorStroke
   /// with the given fields replaced by the non-null parameter values.