feat(train): the person's key on Train's devices (ADR 0030, D15)

TrainKeys holds the one key everything of Train's is sealed with: made
on the first write where the account's keys are, kept in the synced
content_key rows sealed to the account, opened on the person's other
devices. Only keys sealed under Train's own IDs open, so the instance
can't relabel one, and a key the account sealed for another app never
counts; two devices that each made one offline write with the older. A
device without the account's keys opens and writes nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_flutter/test/support/memory_content_key_gateway.dart

@@ -0,0 +1,19 @@
+import 'dart:typed_data';
+
+import 'package:halkyon_train_flutter/src/core/privacy/content_key_gateway.dart';
+
+/// [ContentKeyGateway] in memory, which tests share between "devices" as the
+/// sync engine would, and change as an instance could.
+class MemoryContentKeyGateway() implements ContentKeyGateway {
+  /// The rows, as the instance holds them.
+  final rows = <ContentKeyRow>[];
+
+  @override
+  Future<List<ContentKeyRow>> all() async => List.of(rows);
+
+  @override
+  Future<void> insert({
+    required String dataKeyId,
+    required Uint8List sealedKey,
+  }) async => rows.add((dataKeyId: dataKeyId, sealedKey: sealedKey));
+}