[red] specify file-backed current-draft recovery invariants

Signed-off-by: Viwat Vchirawongkwin <viwat.v@chula.ac.th>

Author: yyods

app/lib/data/draft_repository.dart

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: MIT
+// Part of PyBLE (https://pyble.dev) — see /LICENSE.
+
+/// Neutral local current-text recovery seam (ADR-0048).
+library;
+
+/// One complete, immutable local checkpoint. Origin is historical context,
+/// never authority to write to a board.
+class DraftSnapshot {
+  const DraftSnapshot({
+    required this.name,
+    required this.content,
+    this.originPath,
+    required this.generation,
+    required this.checkpointTime,
+  });
+
+  final String name;
+  final String content;
+  final String? originPath;
+  final int generation;
+  final DateTime checkpointTime;
+}
+
+/// A committed slot, including a high-water generation for an empty tombstone.
+class DraftRead {
+  const DraftRead({required this.generation, this.snapshot});
+  final int generation;
+  final DraftSnapshot? snapshot;
+}
+
+/// Bounded reasons suitable for localized UI; never includes user source/path.
+enum DraftStorageFailure {
+  unavailable,
+  corrupt,
+  unsupportedVersion,
+  invalidMetadata,
+  invalidText,
+  tooLarge,
+  conflict,
+}
+
+class DraftStorageException implements Exception {
+  const DraftStorageException(this.reason);
+  final DraftStorageFailure reason;
+
+  @override
+  String toString() => 'DraftStorageException(${reason.name})';
+}
+
+/// Sole serialized writer for one local slot. A successful Future means that
+/// its matching transaction committed, not that a board was contacted.
+abstract interface class DraftRepository {
+  Future<DraftRead> load();
+  Future<void> write(DraftSnapshot snapshot, {required int expectedGeneration});
+  Future<int> clear({required int expectedGeneration});
+  Future<void> close();
+}

app/lib/data/drift_draft_repository.dart

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+// Part of PyBLE (https://pyble.dev) — see /LICENSE.
+
+import 'dart:io';
+import 'draft_repository.dart';
+
+/// File-backed implementation of the bounded recovery seam.
+class DriftDraftRepository implements DraftRepository {
+  static Future<DriftDraftRepository> open(File file) async =>
+      DriftDraftRepository();
+
+  @override
+  Future<DraftRead> load() async => throw UnimplementedError();
+  @override
+  Future<void> write(DraftSnapshot snapshot, {required int expectedGeneration}) async => throw UnimplementedError();
+  @override
+  Future<int> clear({required int expectedGeneration}) async => throw UnimplementedError();
+  @override
+  Future<void> close() async {}
+}