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/canvas/pen_slide_screen.dart

@@ -15,6 +15,7 @@ import 'package:flutter/material.dart';
 import 'package:path/path.dart' as p;
 import 'package:syncfusion_flutter_pdf/pdf.dart';
 
+import '../engine/brush.dart';
 import '../input/pen_config.dart';
 import '../input/pen_input_service.dart';
 import '../input/pressure_curve.dart' show kNaturalPressureGamma;
@@ -52,6 +53,11 @@ class _PenSlideScreenState extends State<PenSlideScreen> {
   Map<int, Size>? _slideSizes;
 
   CanvasTool _tool = CanvasTool.pen;
+
+  /// Selected brush for the PEN tool. Highlighter tool uses the highlighter
+  /// brush; local state only (not persisted — TODO(brush-persist-selection)).
+  BrushKind _penBrush = BrushKind.fountainPen;
+
   Color _color = Colors.black;
   bool _allowFingerDrawing = false;
   bool _needsCenter = true;
@@ -363,6 +369,7 @@ class _PenSlideScreenState extends State<PenSlideScreen> {
           strokes: _currentStrokes,
           transformationController: _transform,
           tool: _tool,
+          brush: _penBrush,
           color: _color,
           strokeWidth: _strokeWidth,
           pressureGamma:
@@ -396,11 +403,15 @@ class _PenSlideScreenState extends State<PenSlideScreen> {
         child: Row(
           mainAxisSize: MainAxisSize.min,
           children: [
-            ToolButton(
-              icon: Icons.edit_outlined,
-              selected: _tool == CanvasTool.pen,
-              tooltip: 'Pen',
-              onPressed: () => setState(() => _tool = CanvasTool.pen),
+            BrushPickerButton(
+              selected: _penBrush,
+              active: _tool == CanvasTool.pen,
+              tooltip: 'Brush',
+              labelFor: brushLabelEn,
+              onSelected: (b) => setState(() {
+                _penBrush = b;
+                _tool = CanvasTool.pen;
+              }),
             ),
             ToolButton(
               icon: Icons.brush_outlined,