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.

test/export_geometry_test.dart

@@ -9,6 +9,7 @@ import 'dart:ui';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:perfect_freehand/perfect_freehand.dart' as pf;
 
+import 'package:badnote/editor/engine/brush.dart';
 import 'package:badnote/editor/engine/stroke_geometry.dart';
 import 'package:badnote/editor/engine/stroke_model.dart';
 
@@ -37,6 +38,9 @@ void main() {
       isHighlighter: false,
       hasRealPressure: true,
       isComplete: true,
+      // buildStrokeOutline now resolves the stroke's brush into the recipe, so
+      // the reference must pass the SAME brush to still pin "one shared recipe".
+      brush: brushProfileFor(stroke.brush),
     );
     expect(shared, isNotEmpty);