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/live_ink_painter.dart

@@ -0,0 +1,47 @@
+// lib/editor/render/live_ink_painter.dart
+//
+// CustomPainter for the in-progress stroke (live) layer.
+//
+// Paints only the single EditorStroke? currently being drawn, with
+// isComplete:false so perfect_freehand tapers the trailing end correctly.
+// Kept in a separate RepaintBoundary so committed strokes are never
+// re-rasterized on pointer-move events.
+
+import 'package:flutter/material.dart';
+
+import '../engine/stroke_geometry.dart';
+import '../engine/stroke_model.dart';
+
+/// Paints the single in-progress [EditorStroke] (or nothing when [live] is
+/// null / empty). Use alongside [StaticInkPainter] in stacked [CustomPaint]s.
+class LiveInkPainter extends CustomPainter {
+  const LiveInkPainter({
+    required this.live,
+    required this.pageSize,
+  });
+
+  /// The stroke currently being drawn, or null when idle.
+  final EditorStroke? live;
+  final Size pageSize;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final stroke = live;
+    if (stroke == null || stroke.points.isEmpty) return;
+
+    final path = buildStrokeOutline(stroke, pageSize, isComplete: false);
+    if (path.getBounds().isEmpty) return;
+
+    canvas.drawPath(
+      path,
+      Paint()
+        ..color = Color(stroke.color)
+        ..style = PaintingStyle.fill
+        ..isAntiAlias = true,
+    );
+  }
+
+  @override
+  bool shouldRepaint(LiveInkPainter old) =>
+      !identical(old.live, live) || old.pageSize != pageSize;
+}