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/pen_polish_test.dart

@@ -14,6 +14,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 import 'package:badnote/editor/canvas/ink_painters.dart';
 import 'package:badnote/editor/canvas/pen_stroke.dart';
+import 'package:badnote/editor/engine/brush.dart';
 import 'package:badnote/editor/engine/stroke_geometry.dart';
 import 'package:badnote/editor/engine/stroke_model.dart';
 
@@ -44,22 +45,27 @@ void main() {
       expect(byDefault.getBounds(), explicit.getBounds());
     });
 
-    test('thinning actually affects the outline (not hardcoded/ignored)', () {
-      // NB: the bounding box is thinning-INVARIANT here because
-      // perfect_freehand's round end-caps are drawn at the full `size`; only
-      // the mid-section width tracks pressure×thinning. So we compare the
-      // outline PERIMETER (sum of contour lengths), which does reflect the
-      // pinched middle.
+    test('brush thinning actually affects the outline (param is wired)', () {
+      // After the brush-engine rebuild each brush owns its perfect_freehand
+      // thinning (spec §4): fountainPen = 0.9 (pressure-modulated, pinched
+      // middle) vs highlighter = 0.0 (constant full width). The bounding box is
+      // thinning-INVARIANT (round caps at full size), so compare the outline
+      // PERIMETER, which reflects the pinched middle.
       double perimeter(Path p) =>
           p.computeMetrics().fold(0.0, (sum, m) => sum + m.length);
-      final pen = _pressuredPen();
-      final strong = perimeter(
-          buildStrokePath(pen, size, isComplete: true, thinning: 0.85));
-      final none = perimeter(
-          buildStrokePath(pen, size, isComplete: true, thinning: 0.0));
-      // Constant width (0.0) vs pressure-thinning (0.85) must differ measurably.
+      final pressured = _pressuredPen(); // fountainPen brush, thinning 0.9
+      final flat = PenStroke(
+        points: _pressuredPen().points,
+        color: 0xFF000000,
+        width: 0.01,
+        kind: PenStrokeKind.highlighter,
+        brush: BrushKind.highlighter, // highlighter brush, thinning 0.0
+      );
+      final strong =
+          perimeter(buildStrokePath(pressured, size, isComplete: true));
+      final none = perimeter(buildStrokePath(flat, size, isComplete: true));
       expect((strong - none).abs(), greaterThan(1.0),
-          reason: 'thinning had no effect on the outline — it is not wired');
+          reason: 'brush thinning had no effect on the outline — not wired');
     });
 
     test('screen and export builders agree for the same stroke + thinning', () {