Restructure step 0 done: notice table test, flaky-link proxy, liveness on the engine clock, STARSYNC_PROFILE

- notice_table_test: one row per AppEvent variant through noticeFor.
- FlakyProxy (core/testing.dart): pass, blackhole (half-open), delay,
  hold, cut after N bytes, reset; demo tests against today's server.
- Liveness (stale/drop/ping) on the engine clock, so grace can be tested
  with a fake clock; Connection takes an injectable clock.
- AppDirs with STARSYNC_PROFILE=<name>: separate data and logs, so two
  instances run on one machine.
- DEVELOPMENT: scenario list per restructure step; test tags follow the
  new step order.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/test/app_dirs_test.dart

@@ -0,0 +1,36 @@
+// restructure: rewrite in step R (Rename): Hive moves into the support
+// folder for the normal app too.
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:watch_together/src/platform/app_dirs.dart';
+
+void main() {
+  final sep = Platform.pathSeparator;
+  const support = '/s';
+
+  test('no profile: today\'s layout', () {
+    final d = AppDirs.fromEnv(support: support, env: const {});
+    expect(d.profile, isNull);
+    expect(d.hive, isNull, reason: "Hive's default folder");
+    expect(d.logs, '$support${sep}logs');
+  });
+
+  test('a profile gets its own Hive and logs folders', () {
+    final d = AppDirs.fromEnv(support: support, env: const {profileEnv: 'b'});
+    expect(d.profile, 'b');
+    expect(d.hive, '$support${sep}profiles${sep}b${sep}hive');
+    expect(d.logs, '$support${sep}profiles${sep}b${sep}logs');
+    expect(d.support, support, reason: 'yt-dlp and shaders stay shared');
+  });
+
+  test('an unusable name is ignored and reported', () {
+    for (final bad in ['../x', 'a b', 'x' * 33, 'é']) {
+      final env = {profileEnv: bad};
+      expect(AppDirs.fromEnv(support: support, env: env).profile, isNull, reason: bad);
+      expect(AppDirs.invalidProfile(env), bad);
+    }
+    expect(AppDirs.invalidProfile(const {profileEnv: ''}), isNull, reason: 'empty is just unset');
+    expect(AppDirs.invalidProfile(const {profileEnv: 'ok_1-2'}), isNull);
+  });
+}