chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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/learn/halkyon_learn_flutter/test/l10n/arb_files_test.dart

@@ -0,0 +1,78 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// The messages of the `.arb` file [name] in `lib/l10n/`, without metadata.
+Map<String, String> _messages(String name) => {
+  for (final MapEntry(:key, :value) in (jsonDecode(
+    File('lib/l10n/$name').readAsStringSync(),
+  ) as Map<String, Object?>).entries)
+    if (!key.startsWith('@')) key: value! as String,
+};
+
+/// The names of the placeholders in [message], with plural and select
+/// arguments; the words of their branches don't count. (gen-l10n reads
+/// quotes as text here: `use-escaping` is off.)
+Set<String> _placeholders(String message) {
+  final names = <String>{};
+  // Even depths hold text and placeholders, odd ones a plural's branches.
+  var depth = 0;
+  for (var i = 0; i < message.length; i++) {
+    switch (message[i]) {
+      case '{':
+        final name = RegExp(r'\{\s*(\w+)\s*[,}]').matchAsPrefix(message, i);
+        if (name != null && depth.isEven) {
+          names.add(name.group(1)!);
+        }
+        depth++;
+      case '}':
+        depth--;
+    }
+  }
+  return names;
+}
+
+void main() {
+  final template = _messages('app_en.arb');
+
+  test('German has every message, with the same placeholders', () {
+    final german = _messages('app_de.arb');
+
+    expect(
+      template.keys.where((key) => !german.containsKey(key)),
+      isEmpty,
+      reason: 'untranslated',
+    );
+    expect(
+      german.keys.where((key) => !template.containsKey(key)),
+      isEmpty,
+      reason: 'not in the template',
+    );
+    for (final MapEntry(:key, :value) in german.entries) {
+      expect(_placeholders(value), _placeholders(template[key]!), reason: key);
+    }
+  });
+
+  test("the English variants override the template's messages only, with "
+      'the same placeholders', () {
+    for (final variant in ['app_en_GB.arb', 'app_en_US.arb']) {
+      for (final MapEntry(:key, :value) in _messages(variant).entries) {
+        expect(template, contains(key), reason: '$variant: $key');
+        expect(
+          _placeholders(value),
+          _placeholders(template[key]!),
+          reason: '$variant: $key',
+        );
+      }
+    }
+  });
+
+  test('a placeholder is found also inside a plural', () {
+    expect(
+      _placeholders('{count, plural, =1{1 card} other{{count} cards in {t}}}'),
+      {'count', 't'},
+    );
+    expect(_placeholders("Couldn't open {name}"), {'name'});
+  });
+}