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/render/annotation_layer.dart

@@ -0,0 +1,71 @@
+// lib/editor/render/annotation_layer.dart
+//
+// Composites the static committed-stroke layer and the live in-progress layer
+// into a single widget. Wrap the page widget with this to get ink rendering.
+//
+// Layout:
+//   RepaintBoundary
+//     └─ Stack
+//          ├─ CustomPaint(StaticInkPainter)   ← repaints only on revision bump
+//          └─ CustomPaint(LiveInkPainter)      ← repaints on every pointer move
+
+import 'package:flutter/material.dart';
+
+import '../engine/stroke_model.dart';
+import '../engine/stroke_store.dart';
+import 'ink_picture_cache.dart';
+import 'live_ink_painter.dart';
+import 'static_ink_painter.dart';
+
+/// A [StatelessWidget] that renders committed and live ink strokes over a
+/// [pageSize]-sized area.
+///
+/// Place it as an overlay on top of the page content; it is fully transparent
+/// where no strokes are drawn.
+///
+/// [hostId] identifies the ink host (e.g. page id) and is used as the cache
+/// key prefix so multiple pages can share an [InkPictureCache] instance.
+class AnnotationLayer extends StatelessWidget {
+  const AnnotationLayer({
+    super.key,
+    required this.hostId,
+    required this.store,
+    required this.liveStroke,
+    required this.pageSize,
+    required this.cache,
+  });
+
+  final String hostId;
+  final StrokeStore store;
+
+  /// The stroke currently being drawn, or null when idle.
+  final EditorStroke? liveStroke;
+  final Size pageSize;
+  final InkPictureCache cache;
+
+  @override
+  Widget build(BuildContext context) {
+    return RepaintBoundary(
+      child: Stack(
+        children: [
+          CustomPaint(
+            size: pageSize,
+            painter: StaticInkPainter(
+              hostId: hostId,
+              store: store,
+              pageSize: pageSize,
+              cache: cache,
+            ),
+          ),
+          CustomPaint(
+            size: pageSize,
+            painter: LiveInkPainter(
+              live: liveStroke,
+              pageSize: pageSize,
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}