feat(split): scratchpad inks on the pen-first canvas

The split-view scratchpad was the last surface on the old ink_canvas. Move
its inking engine to the performant PenCanvas while keeping the infinite
auto-expanding world.

Key idea: store strokes in absolute WORLD pixels (InkStroke — unchanged
saveScratchpad format) and render through PenCanvas by normalizing against
the CURRENT world size. When the world auto-expands, stored world coords do
not move — only the normalization divisor grows — so ink stays put with zero
drift (proven by the world-expand-stability test).

- Replace InteractiveViewer+SizedBox+InkCanvas with PenCanvas (own pan/zoom,
  minScale 0.1 to survey the big world); keep the AnnotationToolbar.
- Stroke callbacks go through ink_stroke_adapter; load filters to freehand
  so the canvas list stays 1:1 with the undo manager.
- Pen/highlighter/eraser map from the toolbar's PenTool; width is world px.

The left PDF-reference pane (SfPdfViewer, read-only) is unchanged.

Tests: world-expand stability added. flutter analyze: 0. Suite: 270/270.

lib/screens/split_view_screen.dart

@@ -4,7 +4,11 @@ import 'dart:io';
 
 import 'package:flutter/material.dart';
 import 'package:syncfusion_flutter_pdfviewer/pdfviewer.dart';
+import 'package:uuid/uuid.dart';
 
+import '../editor/canvas/pen_canvas.dart';
+import '../editor/canvas/pen_stroke.dart';
+import '../editor/notebook/ink_stroke_adapter.dart';
 import '../models/ink_stroke.dart';
 import '../models/pen_tool.dart';
 import '../models/pressure_curve.dart';
@@ -12,7 +16,6 @@ import '../services/database_service.dart';
 import '../services/undo_manager.dart';
 import '../utils/stroke_stabilizer.dart';
 import '../widgets/annotation_toolbar.dart';
-import '../widgets/ink_canvas.dart';
 
 /// Split-view derivation mode: left pane = reference PDF, right pane = infinite
 /// scratchpad for formula derivation. Scratchpad strokes are persisted per
@@ -43,10 +46,29 @@ class _SplitViewState extends State<SplitViewScreen> {
   bool _isDraggingDivider = false;
 
   // -- Scratchpad (right pane) --
+  // The scratchpad is an infinite WORLD: strokes are stored in absolute world
+  // pixels ([InkStroke], unchanged persistence format), and rendered through the
+  // performant PenCanvas by normalizing against the CURRENT world size. When the
+  // world auto-expands, the stored world coords don't move — only the
+  // normalization divisor grows — so ink stays put with zero drift.
   final UndoManager _undoManager = UndoManager();
   List<InkStroke> _strokes = [];
   double _canvasWidth = 4000;
   double _canvasHeight = 4000;
+  static const _uuid = Uuid();
+
+  /// Pan/zoom transform for the scratchpad world (PenCanvas drives this).
+  final TransformationController _scratchTransform = TransformationController();
+
+  Size get _worldSize => Size(_canvasWidth, _canvasHeight);
+
+  /// Maps the scratchpad toolbar's [PenTool] to the pen-canvas tool. Shapes and
+  /// text fall back to pen (the pen-first scratchpad is freehand).
+  CanvasTool get _canvasTool => switch (_currentTool) {
+        PenTool.eraser => CanvasTool.eraser,
+        PenTool.highlighter => CanvasTool.highlighter,
+        _ => CanvasTool.pen,
+      };
 
   // -- Tool state --
   PenTool _currentTool = PenTool.pen;
@@ -77,6 +99,7 @@ class _SplitViewState extends State<SplitViewScreen> {
     _saveTimer?.cancel();
     _saveImmediate();
     _pdfController.dispose();
+    _scratchTransform.dispose();
     super.dispose();
   }
 
@@ -87,8 +110,11 @@ class _SplitViewState extends State<SplitViewScreen> {
     final strokes = await db.loadScratchpad(widget.documentId);
     if (mounted) {
       setState(() {
-        _strokes = strokes;
-        for (final s in strokes) {
+        // Keep only freehand strokes so the canvas list stays 1:1 with the
+        // undo manager (shapes/text have no pen-canvas representation).
+        final freehand = strokes.where((s) => isFreehandTool(s.tool)).toList();
+        _strokes = freehand;
+        for (final s in freehand) {
           _undoManager.addStroke(s);
         }
       });
@@ -111,7 +137,11 @@ class _SplitViewState extends State<SplitViewScreen> {
 
   // -- Scratchpad stroke callbacks --
 
-  void _onStrokeComplete(InkStroke stroke) {
+  /// PenCanvas committed a stroke (normalized to the current world). Convert it
+  /// to absolute world coords for storage.
+  void _onStrokeComplete(PenStroke pen) {
+    final stroke = inkStrokeFromPen(pen, _worldSize,
+        id: _uuid.v4(), createdAt: DateTime.now());
     setState(() {
       _strokes.add(stroke);
       _undoManager.addStroke(stroke);
@@ -120,13 +150,19 @@ class _SplitViewState extends State<SplitViewScreen> {
     _scheduleSave();
   }
 
-  void _onErase(String strokeId, List<InkStroke> replacements) {
+  /// PenCanvas erased through stroke [index] (into [_strokes]); [replacements]
+  /// are the surviving sub-strokes (normalized) — convert back to world coords.
+  void _onErase(int index, List<PenStroke> replacements) {
+    if (index < 0 || index >= _strokes.length) return;
     setState(() {
-      final original = _strokes.where((s) => s.id == strokeId).firstOrNull;
-      if (original != null) {
-        _undoManager.removeStroke(original, replacements: replacements);
-        _strokes = List.from(_undoManager.currentStrokes);
-      }
+      final original = _strokes[index];
+      final inkReplacements = [
+        for (final r in replacements)
+          inkStrokeFromPen(r, _worldSize,
+              id: _uuid.v4(), createdAt: DateTime.now()),
+      ];
+      _undoManager.removeStroke(original, replacements: inkReplacements);
+      _strokes = List.from(_undoManager.currentStrokes);
     });
     _scheduleSave();
   }
@@ -432,29 +468,24 @@ class _SplitViewState extends State<SplitViewScreen> {
   }
 
   Widget _buildScratchpadPane() {
+    // Render the world through the performant PenCanvas: strokes normalized
+    // against the current world size; toolbar width is in world pixels, so the
+    // pen-canvas fraction is width / worldWidth.
     return Container(
       color: Theme.of(context).scaffoldBackgroundColor,
-      child: InteractiveViewer(
-        constrained: false,
-        minScale: 0.25,
+      child: PenCanvas(
+        pageSize: _worldSize,
+        strokes: penStrokesFromInk(_strokes, _worldSize),
+        transformationController: _scratchTransform,
+        tool: _canvasTool,
+        color: _currentColor,
+        strokeWidth: _currentStrokeWidth / _canvasWidth,
+        // The world is huge, so allow zooming further out to survey it.
+        minScale: 0.1,
         maxScale: 8.0,
-        boundaryMargin: const EdgeInsets.all(double.infinity),
-        child: SizedBox(
-          width: _canvasWidth,
-          height: _canvasHeight,
-          child: InkCanvas(
-            strokes: _strokes,
-            onStrokeComplete: _onStrokeComplete,
-            onErase: _onErase,
-            tool: _currentTool,
-            color: _currentColor,
-            strokeWidth: _currentStrokeWidth,
-            pressureCurve: PressureCurve(type: _pressureCurveType),
-            stabilizationLevel: _stabilizationLevel,
-            filled: _filled,
-            interactionMode: InteractionMode.draw,
-          ),
-        ),
+        onStrokeComplete: _onStrokeComplete,
+        onEraseStroke: _onErase,
+        pageWidget: const ColoredBox(color: Colors.white),
       ),
     );
   }