feat(engine): P0 stroke engine + persistence

Per the full-refactor plan §9 (input-independent half of P0):
- engine: canonical EditorStroke (lossless InkStroke round-trip) +
  stroke_geometry (single getStroke outline) + revision-gated StrokeStore
- render: static/live ink painters + ink_picture_cache (revision-keyed)
  + annotation_layer (RepaintBoundary)
- persistence: DB v6 (ink, notebook_pages) + editor_repository diff-write
  (UPSERT changed / DELETE removed in one txn; id-set after commit) +
  save_scheduler
- pdf_service export now FILLS the getStroke outline (R7 hairline fix)
Not yet wired into the live editor (input relocation pending pen-pressure
diagnostic). 28 new tests pass.

lib/editor/engine/stroke_model.g.dart

@@ -0,0 +1,73 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'stroke_model.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+_$EditorPointImpl _$$EditorPointImplFromJson(Map<String, dynamic> json) =>
+    _$EditorPointImpl(
+      x: (json['x'] as num).toDouble(),
+      y: (json['y'] as num).toDouble(),
+      pressure: (json['pressure'] as num?)?.toDouble(),
+      tilt: (json['tilt'] as num?)?.toDouble(),
+      timestamp: (json['timestamp'] as num?)?.toInt(),
+      pointerDeviceKind: $enumDecodeNullable(
+        _$InputDeviceKindEnumMap,
+        json['pointerDeviceKind'],
+      ),
+    );
+
+Map<String, dynamic> _$$EditorPointImplToJson(_$EditorPointImpl instance) =>
+    <String, dynamic>{
+      'x': instance.x,
+      'y': instance.y,
+      'pressure': instance.pressure,
+      'tilt': instance.tilt,
+      'timestamp': instance.timestamp,
+      'pointerDeviceKind': _$InputDeviceKindEnumMap[instance.pointerDeviceKind],
+    };
+
+const _$InputDeviceKindEnumMap = {
+  InputDeviceKind.touch: 'touch',
+  InputDeviceKind.mouse: 'mouse',
+  InputDeviceKind.stylus: 'stylus',
+  InputDeviceKind.invertedStylus: 'invertedStylus',
+  InputDeviceKind.trackpad: 'trackpad',
+  InputDeviceKind.unknown: 'unknown',
+};
+
+_$EditorStrokeImpl _$$EditorStrokeImplFromJson(Map<String, dynamic> json) =>
+    _$EditorStrokeImpl(
+      id: json['id'] as String,
+      points: (json['points'] as List<dynamic>)
+          .map((e) => EditorPoint.fromJson(e as Map<String, dynamic>))
+          .toList(),
+      tool:
+          $enumDecodeNullable(_$EditorToolEnumMap, json['tool']) ??
+          EditorTool.pen,
+      color: (json['color'] as num?)?.toInt() ?? 0xFF000000,
+      width: (json['width'] as num?)?.toDouble() ?? 0.003,
+      filled: json['filled'] as bool? ?? false,
+      textContent: json['textContent'] as String?,
+      fontSize: (json['fontSize'] as num?)?.toDouble() ?? 14.0,
+    );
+
+Map<String, dynamic> _$$EditorStrokeImplToJson(_$EditorStrokeImpl instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'points': instance.points,
+      'tool': _$EditorToolEnumMap[instance.tool]!,
+      'color': instance.color,
+      'width': instance.width,
+      'filled': instance.filled,
+      'textContent': instance.textContent,
+      'fontSize': instance.fontSize,
+    };
+
+const _$EditorToolEnumMap = {
+  EditorTool.pen: 'pen',
+  EditorTool.highlighter: 'highlighter',
+  EditorTool.eraser: 'eraser',
+};