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_stroke.dart

@@ -6,6 +6,8 @@
 
 import 'package:flutter/foundation.dart';
 
+import '../engine/brush.dart';
+
 /// A single captured sample of a stroke.
 ///
 /// [x]/[y] are normalized to the page rectangle ([0,1]).
@@ -35,6 +37,7 @@ class PenStroke {
     required this.color,
     required this.width,
     required this.kind,
+    this.brush = BrushKind.fountainPen,
   });
 
   /// Normalized points (see [PenPoint]).
@@ -48,4 +51,12 @@ class PenStroke {
   final double width;
 
   final PenStrokeKind kind;
+
+  /// The brush this stroke was drawn with — drives the perfect_freehand
+  /// geometry (thinning/streamline/smoothing/caps) at render time via
+  /// [brushProfileFor]. The pressure pre-warp ([BrushProfile.pressureGamma]) is
+  /// applied at CAPTURE so it is already baked into [points]. Defaults to
+  /// [BrushKind.fountainPen] (the legacy pen visual) so old/loaded strokes keep
+  /// rendering as before.
+  final BrushKind brush;
 }