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_editor_screen.dart

@@ -26,6 +26,7 @@ import 'package:pdfrx/pdfrx.dart';
 
 import '../../l10n/app_localizations.dart';
 import '../../services/database_service.dart';
+import '../engine/brush.dart';
 import '../engine/stroke_eraser.dart';
 import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import '../engine/stroke_model.dart';
@@ -35,7 +36,7 @@ import '../input/diagnostic_logger.dart';
 import '../input/pen_config.dart';
 import '../input/pen_input_service.dart';
 import '../input/pressure_curve.dart'
-    show PressureCurve, kNaturalPressureGamma, kNaturalPressureFloor;
+    show PressureCurve, kNaturalPressureFloor;
 import '../pdf/pen_capture_region.dart';
 import '../persistence/editor_repository.dart';
 import '../persistence/save_scheduler.dart';
@@ -146,6 +147,11 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   // Tool state.
   CanvasTool _tool = CanvasTool.pen;
 
+  /// Selected brush for the PEN tool (fountain/ballpoint/pencil). The
+  /// highlighter tool always uses [BrushKind.highlighter]. Local state only for
+  /// this increment (not persisted — TODO(brush-persist-selection)).
+  BrushKind _penBrush = BrushKind.fountainPen;
+
   /// When true the "select text" tool is active: pen capture is disabled so the
   /// pen falls through to pdfrx for native text selection.
   bool _selectTextMode = false;
@@ -235,6 +241,13 @@ 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,
               ))
           .toList();
     }
@@ -358,9 +371,11 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     final raw = _rawNormalizedPressure(event);
     if (raw == null) return null;
     // PenConfig exposes gamma but not floor; use the shared natural floor (the
-    // bitmap editor did the same — it never sourced floor from config).
+    // bitmap editor did the same — it never sourced floor from config). The
+    // gamma is the BRUSH's pressure warp (fountain p² / pencil √p / linear),
+    // superseding the legacy config gamma — see TODO(brush-pressure-knob).
     const floor = kNaturalPressureFloor;
-    final gamma = _penConfig?.value.pressureGamma ?? kNaturalPressureGamma;
+    final gamma = brushProfileFor(_currentBrush()).pressureGamma;
     return PressureCurve(floor: floor, gamma: gamma).apply(raw);
   }
 
@@ -440,6 +455,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
       color: _currentColor().toARGB32(),
       width: _currentStrokeWidth(),
       kind: _currentKind(),
+      brush: _currentBrush(),
     );
     _bumpOverlay();
   }
@@ -456,6 +472,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
           color: _currentColor().toARGB32(),
           width: _currentStrokeWidth(),
           kind: _currentKind(),
+          brush: _currentBrush(),
         ),
       );
     }
@@ -510,6 +527,12 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
       ? PenStrokeKind.highlighter
       : PenStrokeKind.pen;
 
+  /// Brush in effect: highlighter tool ⇒ highlighter brush, else the selected
+  /// pen brush. Drives both the capture-time pressure warp and render geometry.
+  BrushKind _currentBrush() => _tool == CanvasTool.highlighter
+      ? BrushKind.highlighter
+      : _penBrush;
+
   Color _currentColor() => _tool == CanvasTool.highlighter
       ? _color.withAlpha(0x80)
       : _color;
@@ -754,11 +777,15 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
         child: Row(
           mainAxisSize: MainAxisSize.min,
           children: [
-            ToolButton(
-              icon: Icons.edit_outlined,
-              selected: _tool == CanvasTool.pen && !_selectTextMode,
-              tooltip: l.toolPen,
-              onPressed: () => _setTool(CanvasTool.pen),
+            BrushPickerButton(
+              selected: _penBrush,
+              active: _tool == CanvasTool.pen && !_selectTextMode,
+              tooltip: l.brushPicker,
+              labelFor: (b) => brushLabel(b, l),
+              onSelected: (b) {
+                setState(() => _penBrush = b);
+                _setTool(CanvasTool.pen);
+              },
             ),
             ToolButton(
               icon: Icons.brush_outlined,