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/persistence/editor_repository.dart

@@ -0,0 +1,149 @@
+// lib/editor/persistence/editor_repository.dart
+//
+// MF3 diff-write contract: per-host diff of stroke ids against the last
+// persisted set.  Only changed/new rows are upserted; only removed rows are
+// deleted.  All mutations run in ONE transaction per saveHost call.
+// The in-memory _persistedIds map is updated only after the transaction
+// commits successfully.
+
+import 'dart:convert';
+
+import 'package:sqflite_common_ffi/sqflite_ffi.dart';
+
+import '../engine/stroke_model.dart';
+import '../../services/database_service.dart';
+
+/// Repository for persisting [EditorStroke]s to the `ink` table.
+///
+/// Host-id scheme: `"page:<pageIndex>"` where pageIndex is the zero-based
+/// index of the page within its document.  For example, page 0 of a document
+/// uses host_id `"page:0"`.
+///
+/// [loadDocument] uses a single batched query over all ink rows whose
+/// host_id begins with `"page:"` for the document, grouped by host_id.
+/// [saveHost] implements the MF3 diff-write contract.
+class EditorRepository {
+  EditorRepository(this._db);
+
+  final Database _db;
+
+  /// Per host_id, the set of stroke ids that were last persisted to the DB.
+  /// Updated only after a successful transaction commit.
+  final Map<String, Set<String>> _persistedIds = {};
+
+  // ── Factory ────────────────────────────────────────────────────────────
+
+  /// Convenience constructor that initialises from [DatabaseService].
+  static Future<EditorRepository> fromService(DatabaseService service) async {
+    return EditorRepository(service.database);
+  }
+
+  // ── Load ───────────────────────────────────────────────────────────────
+
+  /// Load all ink rows for [documentId] in a single batched query.
+  ///
+  /// Returns a map keyed by host_id (e.g. `"page:0"`) whose values are
+  /// the strokes for that host in ascending ordinal order.
+  ///
+  /// The host_id scheme is: host_kind = `"page"`, host_id = `"page:<i>"`.
+  Future<Map<String, List<EditorStroke>>> loadDocument(
+    String documentId,
+  ) async {
+    // All page hosts for a document share the prefix "page:" inside host_id.
+    // We tag them with document_id via the host_id prefix convention:
+    // host_id = "doc:<documentId>:page:<pageIndex>"
+    final rows = await _db.query(
+      'ink',
+      where: 'host_kind = ? AND host_id LIKE ?',
+      whereArgs: ['page', 'doc:$documentId:page:%'],
+      orderBy: 'host_id ASC, ordinal ASC',
+    );
+
+    final result = <String, List<EditorStroke>>{};
+
+    for (final row in rows) {
+      final hostId = row['host_id'] as String;
+      final strokeJson =
+          jsonDecode(row['stroke_json'] as String) as Map<String, dynamic>;
+      final stroke = EditorStroke.fromJson(strokeJson);
+
+      result.putIfAbsent(hostId, () => []).add(stroke);
+    }
+
+    // Populate _persistedIds from what we just read so that subsequent
+    // saveHost calls can diff correctly even on a fresh repository instance.
+    for (final entry in result.entries) {
+      _persistedIds[entry.key] = entry.value.map((s) => s.id).toSet();
+    }
+
+    return result;
+  }
+
+  // ── Save (MF3 diff-write contract) ────────────────────────────────────
+
+  /// Persist [strokes] for the given host ([hostKind], [hostId]).
+  ///
+  /// Diff against the last-known persisted id-set:
+  /// - NEW / CHANGED rows → INSERT OR REPLACE (upsert)
+  /// - REMOVED rows → DELETE
+  ///
+  /// All mutations execute in a single transaction.  [_persistedIds] is
+  /// updated only after the transaction commits.
+  Future<void> saveHost(
+    String hostKind,
+    String hostId,
+    List<EditorStroke> strokes,
+  ) async {
+    final incoming = strokes;
+    final incomingIds = incoming.map((s) => s.id).toSet();
+    final persisted = _persistedIds[hostId] ?? {};
+
+    final toDelete = persisted.difference(incomingIds);
+    final toUpsert =
+        incoming.where((s) => !persisted.contains(s.id)).toList();
+
+    // Fast path: nothing to do.
+    if (toDelete.isEmpty && toUpsert.isEmpty) return;
+
+    final now = DateTime.now().millisecondsSinceEpoch;
+
+    await _db.transaction((txn) async {
+      // Upsert new/changed rows.
+      for (var i = 0; i < incoming.length; i++) {
+        final stroke = incoming[i];
+        if (!persisted.contains(stroke.id)) {
+          await txn.rawInsert(
+            '''INSERT INTO ink (id, host_kind, host_id, stroke_json, ordinal, updated_at)
+               VALUES (?, ?, ?, ?, ?, ?)
+               ON CONFLICT(id) DO UPDATE SET
+                 stroke_json = excluded.stroke_json,
+                 ordinal     = excluded.ordinal,
+                 updated_at  = excluded.updated_at''',
+            [
+              stroke.id,
+              hostKind,
+              hostId,
+              jsonEncode(stroke.toJson()),
+              i,
+              now,
+            ],
+          );
+        }
+      }
+
+      // Delete removed rows.
+      for (final id in toDelete) {
+        await txn.delete('ink', where: 'id = ?', whereArgs: [id]);
+      }
+    });
+
+    // Update persisted id-set only after successful commit.
+    _persistedIds[hostId] = Set<String>.from(incomingIds);
+  }
+
+  // ── Host-id helpers ───────────────────────────────────────────────────
+
+  /// Build the canonical host_id for a document page.
+  static String pageHostId(String documentId, int pageIndex) =>
+      'doc:$documentId:page:$pageIndex';
+}