fix(dictionary): no language goes missing while the curator updates

The update removed the files of sources no longer wanted first, Korean's
words from Wiktionary among them, before KRDict's replacement was
written, which on the first run after v0.16.0 came after Wiktionary's
3 GB conversion: the server unloaded Korean meanwhile, and for good if
KRDict failed. Removal now comes last, and a replaced source stays until
its replacement is there. A bare curate update, without --languages,
no longer removes every other language. A KRDict export of another
shape (the page's buttons reordered) is refused rather than written as
a dictionary of no words over Korean's. The server never unloads from
an empty dictionaries folder (a volume made anew), and a failed unload
no longer stops the files from loading.

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

halkyon_learn_curator/bin/curate.dart

@@ -196,6 +196,9 @@ Future<void> main(List<String> args) async {
       Directory(options.option('data')!),
       days,
       languages: _languages(options.option('languages')),
+      // Without --languages, the files of other languages are left as
+      // they are: an operator may keep them up to date otherwise.
+      removesOthers: options.option('languages') != null,
     );
     return;
   }
@@ -536,6 +539,7 @@ Future<void> _update(
   Directory data,
   int? every, {
   Set<String> languages = const {},
+  bool removesOthers = true,
 }) async {
   final (:wiktionary, :chinese, :korean, :greek) = dictionarySources(languages);
   void say(String message) =>
@@ -599,17 +603,22 @@ Future<void> _update(
         ),
       );
   // In this order: Korean's words and Hanja take what Wiktionary kept.
+  // What is no longer wanted goes last, once what replaces it is there.
   Future<bool> languagesOnce() async {
-    final removed = removeUnwanted(into, languages);
+    final done =
+        !(!await chineseOnce() |
+            !await wiktionaryOnce() |
+            !await koreanOnce() |
+            !await charactersOnce());
+    final removed = removesOthers
+        ? removeUnwanted(into, languages)
+        : const <File>[];
     if (removed.isNotEmpty) {
       say(
         'Removed ${[for (final file in removed) file.uri.pathSegments.last].join(', ')}.',
       );
     }
-    return !(!await chineseOnce() |
-        !await wiktionaryOnce() |
-        !await koreanOnce() |
-        !await charactersOnce());
+    return done;
   }
 
   if (every == null) {