build: Melos, a Dart check and every path for the single repository

- Melos 8.9 as the root's dev dependency, with scripts across the
  workspace (analyze, test:dart, test:flutter, generate, check) and its
  filters (--diff, --include-dependents); cli_util overridden to 0.5.2
  for it, since Learn's flutter_launcher_icons asks for 0.4 and uses only
  its logger, unchanged in 0.5.
- tool/check.dart in place of tool/check.sh: packages, formatting of our
  own files, analysis, the tools' tests, then every package's tests
  through Melos (--changed-since=<commit> for only what changed and what
  depends on it). tool/pre-push is the hook: the signature check, then
  the checks. CI generates every server's code and runs the same check.
- The images build from the root with a workspace narrowed to what they
  need (tool/workspace_only.sh); compose, the deploy's migration test,
  the snap, the release script, the editor's launch files and Learn's
  asset tools (run from apps/learn) follow the new layout.
- The project's source is named as project-halkyon/halkyon (source link,
  snap, F-Droid listing, release uploads); the AGPL packages carry the
  licence's own text.

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

test/check_files_test.dart

@@ -0,0 +1,57 @@
+import 'package:test/test.dart';
+
+import '../tool/src/check_files.dart';
+
+void main() {
+  test("our own Dart files are formatted: the apps', the shared packages', the "
+      "tools'", () {
+    for (final path in [
+      'apps/learn/halkyon_learn_flutter/lib/main.dart',
+      'apps/learn/halkyon_learn_server/lib/server.dart',
+      'apps/learn/halkyon_learn_server/test/integration/auth/account_test.dart',
+      'apps/todo/halkyon_todo_flutter/lib/main.dart',
+      'packages/halkyon_app/lib/halkyon_app.dart',
+      'packages/halkyon_base_server/lib/src/auth/email.dart',
+      'tool/check.dart',
+    ]) {
+      expect(isOwnDartFile(path), isTrue, reason: path);
+    }
+  });
+
+  test('what a generator writes is left as it wrote it', () {
+    for (final path in [
+      // The client packages, Serverpod's from top to bottom.
+      'apps/learn/halkyon_learn_client/lib/src/protocol/client.dart',
+      'apps/todo/halkyon_todo_client/lib/halkyon_todo_client.dart',
+      'packages/halkyon_base_client/lib/src/protocol/protocol.dart',
+      // A server's generated sources and test tools.
+      'apps/learn/halkyon_learn_server/lib/src/generated/protocol.dart',
+      'packages/halkyon_base_server/lib/src/generated/endpoints.dart',
+      'apps/learn/halkyon_learn_server/test/integration/test_tools/serverpod_test_tools.dart',
+      // gen-l10n's localizations, the app's and the shared packages'.
+      'apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations.dart',
+      'apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_de.dart',
+      'packages/halkyon_ui/lib/l10n/halkyon_ui_localizations_en.dart',
+      'packages/halkyon_app/lib/l10n/halkyon_app_localizations.dart',
+    ]) {
+      expect(isOwnDartFile(path), isFalse, reason: path);
+    }
+  });
+
+  test('the files of other languages are no Dart files', () {
+    expect(isOwnDartFile('apps/learn/tool/app_icons.py'), isFalse);
+    expect(
+      isOwnDartFile('packages/halkyon_app/lib/l10n/halkyon_app_en.arb'),
+      isFalse,
+    );
+  });
+
+  test("Melos' filters take only what changed since a commit, and what depends "
+      'on it, when asked', () {
+    expect(melosFilters(), isEmpty);
+    expect(melosFilters(changedSince: 'origin/main'), [
+      '--diff=origin/main',
+      '--include-dependents',
+    ]);
+  });
+}