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/ink_painters.dart

@@ -8,8 +8,9 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:perfect_freehand/perfect_freehand.dart' as pf;
 
+import '../engine/brush.dart';
 import '../engine/stroke_geometry.dart'
-    show kDefaultPenThinning, kPenSmoothing, kPenStreamline;
+    show freehandOutlinePoints, kDefaultPenThinning;
 import 'pen_stroke.dart';
 
 /// Builds a filled outline [Path] for one stroke (already scaled to pixels).
@@ -39,22 +40,19 @@ Path buildStrokePath(
       )
       .toList();
 
-  final outline = pf.getStroke(
-    pfPoints,
-    options: pf.StrokeOptions(
-      size: pixelWidth,
-      // Highlighter keeps a constant width (no thinning); pen uses the
-      // configurable [thinning] so real 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. (perfect_freehand 2.x honors REAL pressure when
-      // simulatePressure is false — 1.0.4 ignored it, which made width
-      // unresponsive to pen force.)
-      simulatePressure: !hasRealPressure && !isHighlighter,
-      isComplete: isComplete,
-    ),
+  // Route through THE shared recipe (stroke_geometry.freehandOutlinePoints) so
+  // this PDF-overlay path and the note/slide path can never diverge (R7), and
+  // resolve the stroke's brush so each brush renders with its own
+  // thinning/streamline/smoothing/caps (spec §4). Pressure was already
+  // pre-warped by the brush's gamma at capture, so it is baked into pfPoints.
+  final outline = freehandOutlinePoints(
+    pfPoints: pfPoints,
+    size: pixelWidth,
+    isHighlighter: isHighlighter,
+    hasRealPressure: hasRealPressure,
+    isComplete: isComplete,
+    thinning: thinning,
+    brush: brushProfileFor(stroke.brush),
   );
 
   final path = Path();