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/engine/stroke_geometry.dart

@@ -11,6 +11,7 @@ import 'dart:ui';
 
 import 'package:perfect_freehand/perfect_freehand.dart' as pf;
 
+import 'brush.dart';
 import 'stroke_model.dart';
 
 /// Canonical default for perfect_freehand's `thinning` (how strongly pressure
@@ -51,23 +52,69 @@ List<Offset> freehandOutlinePoints({
   required bool hasRealPressure,
   required bool isComplete,
   double thinning = kDefaultPenThinning,
+  BrushProfile? brush,
 }) {
   if (pfPoints.isEmpty) return const <Offset>[];
   return pf.getStroke(
     pfPoints,
-    options: pf.StrokeOptions(
-      size: size,
-      // Highlighter keeps a constant width (no thinning); pen uses the
-      // configurable [thinning] so Surface-Pen pressure changes width.
-      thinning: isHighlighter ? 0.0 : thinning,
-      smoothing: kPenSmoothing,
-      streamline: kPenStreamline,
-      // Real stylus pressure -> don't simulate; no pressure -> let freehand
-      // fake it based on velocity (highlighter never simulates). perfect_freehand
-      // 2.x honors real pressure when simulatePressure is false.
-      simulatePressure: !hasRealPressure && !isHighlighter,
-      isComplete: isComplete,
+    options: brush != null
+        // Brush-driven path: every geometry knob (thinning / streamline /
+        // smoothing / caps / taper / simulatePressure) comes from the
+        // BrushProfile so each brush renders distinctly. Pressure was already
+        // pre-warped by the brush's gamma at CAPTURE (PressureCurve), so the
+        // pre-warp is baked into pfPoints — perfect_freehand stays linear here.
+        // simulatePressure is forced true only when the device gave us NO real
+        // pressure, so velocity-thinning still kicks in for mice/trackpads.
+        ? _optionsFromBrush(brush,
+            size: size,
+            isComplete: isComplete,
+            hasRealPressure: hasRealPressure)
+        : pf.StrokeOptions(
+            size: size,
+            // Highlighter keeps a constant width (no thinning); pen uses the
+            // configurable [thinning] so Surface-Pen pressure changes width.
+            thinning: isHighlighter ? 0.0 : thinning,
+            smoothing: kPenSmoothing,
+            streamline: kPenStreamline,
+            // Real stylus pressure -> don't simulate; no pressure -> let
+            // freehand fake it based on velocity (highlighter never simulates).
+            // perfect_freehand 2.x honors real pressure when simulatePressure
+            // is false.
+            simulatePressure: !hasRealPressure && !isHighlighter,
+            isComplete: isComplete,
+          ),
+  );
+}
+
+/// Build perfect_freehand [pf.StrokeOptions] from a [BrushProfile] (spec §4).
+///
+/// TODO(brush-opacity): [BrushProfile.opacity] / [BrushProfile.blendMultiply]
+/// are NOT consumed here — geometry only this increment. The caller still paints
+/// fill color (with its own alpha) and BlendMode.srcOver; per-stroke opacity /
+/// BlendMode.multiply for ballpoint/pencil/highlighter lands later.
+pf.StrokeOptions _optionsFromBrush(
+  BrushProfile brush, {
+  required double size,
+  required bool isComplete,
+  required bool hasRealPressure,
+}) {
+  return pf.StrokeOptions(
+    size: size,
+    thinning: brush.pfThinning,
+    smoothing: brush.pfSmoothing,
+    streamline: brush.pfStreamline,
+    // Honor REAL pressure (already gamma-pre-warped at capture). Only fall back
+    // to velocity simulation when the device reported no usable pressure.
+    simulatePressure: brush.simulatePressure || !hasRealPressure,
+    start: pf.StrokeEndOptions.start(
+      cap: brush.capStart,
+      taperEnabled: brush.taper,
     ),
+    end: pf.StrokeEndOptions.end(
+      cap: brush.capEnd,
+      taperEnabled: brush.taper,
+    ),
+    isComplete: isComplete,
   );
 }
 
@@ -101,6 +148,11 @@ Path buildStrokeOutline(
       )
       .toList();
 
+  // Resolve the brush so each stroke renders with its own geometry. The
+  // pressure pre-warp ([BrushProfile.pressureGamma]) was already applied at
+  // capture, so it is baked into the points here.
+  final brush = brushProfileFor(stroke.brush);
+
   final outline = freehandOutlinePoints(
     pfPoints: pfPoints,
     size: stroke.width * pageSize.width,
@@ -108,6 +160,7 @@ Path buildStrokeOutline(
     hasRealPressure: stroke.points.any((p) => p.pressure != null),
     isComplete: isComplete,
     thinning: thinning,
+    brush: brush,
   );
 
   if (outline.isEmpty) return path;