feat(train): the person's workouts, sealed and synced (ADR 0030, D14)

WorkoutRepository keeps a workout with its series, sealed with the
person's key: a workout for its row, a series chunk for its row and its
workout, so the instance can neither read them nor move a chunk to
another workout. A readable or changed row shows locked, never as the
person's; a device without the account's keys saves nothing. On the
device's database a workout and its series are written in one
transaction, and a deleted one is blanked before it goes, so the sync
engine's tombstones hold 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_workout_gateway.dart

@@ -0,0 +1,58 @@
+import 'dart:async';
+
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_train_flutter/src/feature/workout/data/workout_gateway.dart';
+
+/// [WorkoutGateway] in memory, whose rows a test changes as an instance
+/// could; it records the envelopes written, so a test sees what reached it.
+/// Blanking before a delete is the sync gateway's.
+class MemoryWorkoutGateway() implements WorkoutGateway {
+  final workoutRows = <String, Envelope>{};
+  final seriesRows = <SeriesRow>[];
+
+  /// Every envelope ever written, in order.
+  final written = <Envelope>[];
+
+  final _remote = StreamController<void>.broadcast();
+
+  /// As a sync bringing others' changes.
+  void bringRemoteChanges() => _remote.add(null);
+
+  @override
+  Future<List<WorkoutRow>> workouts() async => [
+    for (final MapEntry(:key, :value) in workoutRows.entries)
+      (id: key, envelope: value),
+  ];
+
+  @override
+  Future<List<SeriesRow>> seriesOf(String workoutId) async => [
+    for (final row in seriesRows)
+      if (row.workoutId == workoutId) row,
+  ];
+
+  @override
+  Future<void> insert(WorkoutRow workout, List<SeriesRow> series) async {
+    workoutRows[workout.id] = workout.envelope;
+    seriesRows.addAll(series);
+    written
+      ..add(workout.envelope)
+      ..addAll(series.map((row) => row.envelope));
+  }
+
+  @override
+  Future<void> update(String id, Envelope envelope) async {
+    if (workoutRows.containsKey(id)) {
+      workoutRows[id] = envelope;
+      written.add(envelope);
+    }
+  }
+
+  @override
+  Future<void> delete(String id) async {
+    seriesRows.removeWhere((row) => row.workoutId == id);
+    workoutRows.remove(id);
+  }
+
+  @override
+  Stream<void> get remoteChanges => _remote.stream;
+}