fix: PDF finger ink, chrome UX, OneNote pens, and pen physics

Wire finger drawing on PDF without breaking pinch; auto-hide page scrubber and fix bounce; share sticky tools with resize and per-page remember; side-button select; separate pen slots with colors; rnote pressure shapes plus tip-velocity width and lower stroke latency.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/engine/pen_physics.dart

@@ -0,0 +1,33 @@
+// lib/editor/engine/pen_physics.dart
+//
+// Simple physical tip model: modulate stroke width by tip velocity so fountain
+// ink feels slightly thinner when moving fast (starvation), while ballpoint
+// stays nearly velocity-invariant.
+//
+// TODO(pen-physics-wire): wired at capture in PenCanvas._toNormalized via
+// tip velocity × pressure. PDF editor path still uses brush gamma only.
+
+import 'brush.dart';
+
+/// Modulate width fraction by tip velocity (page-normalized units per second).
+///
+/// Fountain: faster → slightly thinner (ink starvation feel).
+/// Ballpoint: nearly ignore velocity.
+/// Pencil: mild thinning at speed.
+/// Highlighter: ignore velocity (flat marker).
+double tipVelocityWidthScale(BrushKind kind, double speedNormPerSec) {
+  final speed =
+      speedNormPerSec.isNaN || speedNormPerSec < 0 ? 0.0 : speedNormPerSec;
+  // Reference: ~2 page-widths/sec ≈ fast handwriting; clamp influence to [0,1].
+  final t = (speed / 2.0).clamp(0.0, 1.0);
+  switch (kind) {
+    case BrushKind.fountainPen:
+      return 1.0 - 0.15 * t;
+    case BrushKind.ballpoint:
+      return 1.0 - 0.02 * t;
+    case BrushKind.pencil:
+      return 1.0 - 0.08 * t;
+    case BrushKind.highlighter:
+      return 1.0;
+  }
+}