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/static_ink_painter.dart

@@ -0,0 +1,69 @@
+// lib/editor/render/static_ink_painter.dart
+//
+// CustomPainter for the committed-stroke (static) layer.
+//
+// paint() gets-or-builds a ui.Picture of all committed strokes keyed by
+// store.revision, then delegates to canvas.drawPicture — so as long as the
+// revision is unchanged the raster thread replays the same display list at
+// zero CPU cost.
+//
+// shouldRepaint() is O(1): it compares the revision int and pageSize only.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/material.dart';
+
+import '../engine/stroke_geometry.dart';
+import '../engine/stroke_store.dart';
+import 'ink_picture_cache.dart';
+
+/// Paints the committed ink layer by recording strokes into a [ui.Picture]
+/// once per [StrokeStore.revision] and caching it in [InkPictureCache].
+///
+/// Place this inside a [RepaintBoundary] / [CustomPaint] pair. The sibling
+/// [LiveInkPainter] handles the in-progress stroke in a separate layer.
+class StaticInkPainter extends CustomPainter {
+  StaticInkPainter({
+    required this.hostId,
+    required this.store,
+    required this.pageSize,
+    required this.cache,
+  }) : revision = store.revision;
+
+  final String hostId;
+  final StrokeStore store;
+  final Size pageSize;
+  final InkPictureCache cache;
+
+  /// Revision snapshot captured at construction time. Used by [shouldRepaint]
+  /// so two painters built at different revisions compare correctly even when
+  /// they share the same [StrokeStore] instance.
+  final int revision;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final picture = cache.getOrBuild(hostId, store.revision, pageSize, () {
+      final recorder = ui.PictureRecorder();
+      final rec = Canvas(recorder);
+      for (final stroke in store.committed) {
+        final path =
+            buildStrokeOutline(stroke, pageSize, isComplete: true);
+        if (path.getBounds().isEmpty) continue;
+        rec.drawPath(
+          path,
+          Paint()
+            ..color = Color(stroke.color)
+            ..style = PaintingStyle.fill
+            ..isAntiAlias = true,
+        );
+      }
+      return recorder.endRecording();
+    });
+
+    canvas.drawPicture(picture);
+  }
+
+  @override
+  bool shouldRepaint(StaticInkPainter old) =>
+      old.revision != store.revision || old.pageSize != pageSize;
+}