feat(train): interval plans, synced and sealed, with an editor and templates (ADR 0030, D42)

Train's server gains the synced `plan` table, a sealed payload alone;
the migration `plans` only adds it, its space index and the foreign key
to its sync space, and a person's deletion takes it with their space.

The app keeps plans as `plan` rows sealed for their row: a name, what
they are for, whether a step off target is told, and blocks of steps
repeated, each ending after a time, a distance or a lap tap and aiming
at a pace, speed, heart rate or zone, or the rower's split, watts or
stroke rate. Home's Plans tab lists them, a new one starts from nothing
or from a template (4×4 minutes, Tabata, 10×400 m), the editor changes
steps on a sheet, and a deletion asks first and can be undone.

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_plan_gateway.dart

@@ -0,0 +1,35 @@
+import 'dart:async';
+
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_train_flutter/src/feature/plan/data/plan_gateway.dart';
+
+/// [PlanGateway] in memory, whose rows a test changes as an instance could.
+class MemoryPlanGateway() implements PlanGateway {
+  final rows = <String, Envelope>{};
+  final _remote = StreamController<void>.broadcast();
+
+  /// As a sync bringing others' changes.
+  void bringRemoteChanges() => _remote.add(null);
+
+  @override
+  Future<List<PlanRow>> plans() async => [
+    for (final MapEntry(:key, :value) in rows.entries)
+      (id: key, envelope: value),
+  ];
+
+  @override
+  Future<void> insert(PlanRow plan) async => rows[plan.id] = plan.envelope;
+
+  @override
+  Future<void> update(String id, Envelope envelope) async {
+    if (rows.containsKey(id)) {
+      rows[id] = envelope;
+    }
+  }
+
+  @override
+  Future<void> delete(String id) async => rows.remove(id);
+
+  @override
+  Stream<void> get remoteChanges => _remote.stream;
+}