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/notebook/ink_stroke_adapter.dart

@@ -19,6 +19,7 @@ import '../../models/ink_point.dart';
 import '../../models/ink_stroke.dart';
 import '../../models/pen_tool.dart';
 import '../canvas/pen_stroke.dart';
+import '../engine/brush.dart';
 
 /// Logical page rectangle a blank note is inked on (portrait, ~A4 √2 ratio).
 /// Strokes are normalized against this so they stay pinned under zoom/pan.
@@ -55,6 +56,12 @@ PenStroke? penStrokeFromInk(InkStroke s, Size page) {
     color: s.color,
     width: s.strokeWidth / w,
     kind: penKindFromTool(s.tool),
+    // Brush isn't persisted yet (TODO(brush-persist)); derive from the tool so
+    // a loaded highlighter renders with the flat highlighter brush and pens
+    // fall back to the fountainPen default.
+    brush: s.tool == PenTool.highlighter
+        ? BrushKind.highlighter
+        : BrushKind.fountainPen,
   );
 }