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_canvas.dart

@@ -21,6 +21,7 @@
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 
+import '../engine/brush.dart';
 import '../engine/stroke_eraser.dart';
 import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import '../engine/stroke_model.dart';
@@ -47,6 +48,7 @@ class PenCanvas extends StatefulWidget {
     required this.strokes,
     required this.transformationController,
     required this.tool,
+    this.brush = BrushKind.fountainPen,
     required this.color,
     required this.strokeWidth,
     required this.onStrokeComplete,
@@ -84,6 +86,13 @@ class PenCanvas extends StatefulWidget {
   final TransformationController transformationController;
 
   final CanvasTool tool;
+
+  /// The brush selected for the PEN tool (fountain/ballpoint/pencil). The
+  /// highlighter tool always renders with [BrushKind.highlighter] regardless of
+  /// this value; the eraser draws nothing. Drives both the capture-time pressure
+  /// pre-warp ([BrushProfile.pressureGamma]) and the render geometry.
+  final BrushKind brush;
+
   final Color color;
 
   /// Pen width as a fraction of page width (so it zooms with the page).
@@ -111,6 +120,12 @@ class PenCanvas extends StatefulWidget {
   /// Pressure-response exponent applied to raw stylus pressure BEFORE it reaches
   /// perfect_freehand. <1 boosts light touches (responsive, rnote-like); 1 is
   /// raw linear (the old "pressure-finger" feel). From `PenConfig.pressureGamma`.
+  ///
+  /// TODO(brush-pressure-knob): superseded by the per-brush
+  /// [BrushProfile.pressureGamma] (fountain p², pencil √p) which now drives the
+  /// capture-time warp. This config knob is retained for the API + future
+  /// reconciliation (e.g. a user multiplier on top of the brush curve) but is no
+  /// longer read by [_normalizedPressure].
   final double pressureGamma;
 
   /// Minimum shaped pressure, so a light stroke still has body instead of
@@ -204,6 +219,16 @@ class _PenCanvasState extends State<PenCanvas> {
 
   bool _isStylus(PointerDeviceKind kind) => arbiter.isStylusKind(kind);
 
+  /// The brush in effect for the current tool: highlighter tool ⇒ highlighter
+  /// brush, otherwise the selected pen brush. (Eraser draws nothing, so its
+  /// brush is irrelevant.)
+  BrushKind get _currentBrush => widget.tool == CanvasTool.highlighter
+      ? BrushKind.highlighter
+      : widget.brush;
+
+  /// The brush profile in effect, for the capture-time pressure pre-warp.
+  BrushProfile get _currentBrushProfile => brushProfileFor(_currentBrush);
+
   /// Normalize stylus pressure to [0,1], or null when the device reports no
   /// usable pressure range (then perfect_freehand simulates pressure).
   ///
@@ -215,8 +240,16 @@ class _PenCanvasState extends State<PenCanvas> {
     if (!_isStylus(event.kind)) return null;
     final double? raw = _rawNormalizedPressure(event);
     if (raw == null) return null;
-    return PressureCurve(floor: widget.pressureFloor, gamma: widget.pressureGamma)
-        .apply(raw);
+    // Pre-warp pressure with the BRUSH's gamma (rnote PressureCurve: fountain
+    // = Pow2/p², pencil = Sqrt/√p, ballpoint/highlighter = Linear), reusing the
+    // existing PressureCurve. Baking the warp in at capture means the live
+    // stroke and the export replay identical pressures (no divergence). The
+    // brush gamma supersedes the legacy per-config `pressureGamma` knob — see
+    // TODO(brush-pressure-knob) on `widget.pressureGamma`.
+    return PressureCurve(
+      floor: widget.pressureFloor,
+      gamma: _currentBrushProfile.pressureGamma,
+    ).apply(raw);
   }
 
   /// Raw [0,1] stylus force before response shaping (see [_normalizedPressure]).
@@ -378,6 +411,7 @@ class _PenCanvasState extends State<PenCanvas> {
           color: _currentColor().toARGB32(),
           width: widget.strokeWidth,
           kind: _currentKind(),
+          brush: _currentBrush,
         ),
       );
     }
@@ -402,6 +436,7 @@ class _PenCanvasState extends State<PenCanvas> {
         color: _currentColor().toARGB32(),
         width: widget.strokeWidth,
         kind: _currentKind(),
+        brush: _currentBrush,
       );
     });
   }