i18n: missing translations are reported, never a failure (#2729)

A message a catalog lacks, or translates as an empty string, shows in
English and fails nothing:

- build.rs no longer requires every formats catalog to carry the same
  messages, and tr_format! gets a last arm that formats the English
  source, so a message no catalog has builds.
- Coverage tests (every language translates X) and tests of one
  language's exact wording are gone; the language-switching, layout,
  date, placeholder and user-value tests read the catalog's own value
  instead of a hard-coded translation.
- i18n::tests::translation_gaps_report lists, per language, the plain
  and format messages it lacks and its untranslated What's New lines.
  It always passes; cargo xtask ci
  runs it from the test step's binary after the tests.
- catalogs_load checks that every catalog file is a JSON object of
  strings with each key once; it.json named "Dimensions" twice.

apps/lightcraft/src/native_menu.rs

@@ -456,10 +456,6 @@ mod tests {
             set_language(*language);
             assert!(seen.insert(structure_of(&bar)), "{language:?} shares another language's menu structure key");
         }
-        set_language(Locale::ZhHant);
-        assert_eq!(label_text("File"), "檔案");
-        assert_eq!(label_text("Settings…"), "設定…");
-        assert_eq!(label_text("Quit LightCraft"), "結束 LightCraft");
         set_language(Locale::En);
     }