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/intro-paragraph/config.yaml

@@ -4,21 +4,30 @@ prefixFields: true
 prefixType: vendor
 
 fields:
-  - identifier: header
-    useExistingField: true
-    required: true
+  - identifier: header_section
+    type: Palette
+    label: Header
+    fields:
+      - identifier: header
+        useExistingField: true
+        required: true
+      - type: Linebreak
+      - identifier: header_layout
+        useExistingField: true
+      - identifier: header_position
+        useExistingField: true
+      - identifier: date
+        useExistingField: true
+      - type: Linebreak
+      - identifier: header_link
+        useExistingField: true
+      - type: Linebreak
+      - identifier: subheader
+        useExistingField: true
 
-  - identifier: header_layout
-    useExistingField: true
 
-  - identifier: header_position
-    useExistingField: true
 
-  - identifier: header_link
-    useExistingField: true
 
-  - identifier: subheader
-    useExistingField: true
 
   - identifier: bodytext
     useExistingField: true