Checkpoint: headless JSON architecture documented + cleanup (Stufe 1+2)

Restore point before Stufe 3 (central resolver-service refactoring).

Includes this session's work:
- Success Story (usecase) rebuild: new fields/tabs, UsecaseSerializer,
  thin List/Show renderers, MM relations, inline content elements.
- vitec_columns content block (two-column layout with per-item content)
  incl. unified header section; special-cased JSON resolution.
- Container per-column flex (items[].config align/justify) + gap on parent.
- Unified header section across CEs/plugins/containers; header fields in JSON.
- ISO-style architecture spec: Documentation/Headless-JSON-Architecture.md.
- Cleanup: removed local scratch + verified .bak backups.
- Fixes: B-6 (undefined thumbnail variable in Downloadcardcollection image
  resolver), B-7 (unsatisfiable legacy CType OR branch in Downloadcard/Datasheets).

Rollback: git reset --hard snapshot-2026-07-09-pre-stufe3

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/vitec/ContentBlocks/ContentElements/cta-banner/language/labels.xlf

@@ -6,7 +6,7 @@
         <source>VITEC · CTA Banner</source>
       </trans-unit>
       <trans-unit id="description">
-        <source>Call-to-Action Banner mit optionalem zweiten Button und Background-Varianten</source>
+        <source>Call-to-action banner with optional secondary button and background variants</source>
       </trans-unit>
 
       <trans-unit id="primary_cta.label">
@@ -35,7 +35,7 @@
         <source>Background Image (optional)</source>
       </trans-unit>
       <trans-unit id="background_image.description">
-        <source>Optionales Hintergrundbild — überlagert die Background-Variant-Farbe</source>
+        <source>Optional background image — overlays the background variant colour</source>
       </trans-unit>
     </body>
   </file>