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/services/pdf_service.dart

@@ -308,6 +308,12 @@ class PdfService {
     // ONE shared recipe with the on-screen painter (R7): export can no longer
     // drift from screen. Previously this hardcoded thinning:0.7/streamline:0.5,
     // which diverged from the screen's 0.85/0.32 → hairline export mismatch.
+    //
+    // TODO(brush-persist) / TODO(brush-export): InkStroke does not persist the
+    // brush, so export can only use the legacy (no-brush) recipe — it does NOT
+    // yet pass `brush:` here. Once the brush is persisted on InkStroke, resolve
+    // brushProfileFor(stroke.brush) and pass it so export matches the brush-aware
+    // screen geometry (taper/caps/per-brush thinning) for non-fountain brushes.
     final outline = freehandOutlinePoints(
       pfPoints: pfPoints,
       size: pixelWidth,