i18n: missing translations never fail; xtask ci prints one report (#3856)

* i18n tests: report missing translations instead of failing

The scans for untranslated menu strings, tl! literals, undo labels, menu labels and status
messages print what a complete language lacks. A missing translation shows the English
text until a translator adds the row, so it no longer fails the gate.

* i18n: one untranslated report in xtask ci; drop one-language wording tests

`i18n::tests::untranslated_report` lists, per language, the menu strings, tl! literals, undo
step names and templates, menu labels, render-site labels, status and error messages,
PostScript error meanings and default swatch names its catalog lacks; strings every language
lacks are listed once. It never fails on a gap, only on a scan that finds too few strings.
`cargo xtask ci` runs it in an `i18n-report` step, and `cargo xtask i18n-report` runs it alone.

The five scan tests, the render-site and swatch coverage tests and untranslated_messages.txt
fold into the report. Tests that pinned one language's wording are gone; where they also
checked code (lookups, plural rules, locale tags, menu and History step names, engine
parameter errors), that check stays and compares with the catalog lookup.

apps/vectorcraft/src/mac_save_format.rs

@@ -243,12 +243,4 @@ mod tests {
             assert_eq!(fileio::format_for_name(&name).map(|f| f.label), Some(label.as_str()), "{name}");
         }
     }
-
-    /// The menu's label is translated in every language with complete menus.
-    #[test]
-    fn the_label_is_translated() {
-        for lang in i18n::Lang::all().filter(|l| l.complete_menus()) {
-            assert!(i18n::has(lang, "Format:"), "{}", lang.code());
-        }
-    }
 }