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/Configuration/Sets/Vitecset/setup.typoscript

@@ -20,7 +20,7 @@ config {
 # =============================================================================
 
 tt_content {
-    vitec_productlist < lib.contentElement
+    vitec_productlist < lib.contentElementWithHeader
     vitec_productlist {
         fields {
             content {
@@ -32,7 +32,28 @@ tt_content {
         }
     }
 
-    vitec_productshow < lib.contentElement
+    news_pi1 < lib.contentElementWithHeader
+    news_pi1 {
+        fields {
+            content {
+                fields {
+                    news = USER
+                    news.userFunc = Evomedien\Vitec\UserFunc\NewsJsonRenderer->render
+                }
+            }
+        }
+    }
+
+    news_newsliststicky < tt_content.news_pi1
+    news_newsselectedlist < tt_content.news_pi1
+    news_newsdetail < tt_content.news_pi1
+    news_newsdatemenu < tt_content.news_pi1
+    news_categorylist < tt_content.news_pi1
+    news_newssearchform < tt_content.news_pi1
+    news_newssearchresult < tt_content.news_pi1
+    news_taglist < tt_content.news_pi1
+
+    vitec_productshow < lib.contentElementWithHeader
     vitec_productshow {
         fields {
             content {
@@ -44,7 +65,7 @@ tt_content {
         }
     }
 
-    vitec_usecaselist < lib.contentElement
+    vitec_usecaselist < lib.contentElementWithHeader
     vitec_usecaselist {
         fields {
             content {
@@ -56,7 +77,7 @@ tt_content {
         }
     }
 
-    vitec_usecaseshow < lib.contentElement
+    vitec_usecaseshow < lib.contentElementWithHeader
     vitec_usecaseshow {
         fields {
             content {
@@ -68,7 +89,7 @@ tt_content {
         }
     }
 
-    vitec_marketshow < lib.contentElement
+    vitec_marketshow < lib.contentElementWithHeader
     vitec_marketshow {
         fields {
             content {
@@ -80,7 +101,7 @@ tt_content {
         }
     }
 
-    vitec_solutionshow < lib.contentElement
+    vitec_solutionshow < lib.contentElementWithHeader
     vitec_solutionshow {
         fields {
             content {
@@ -92,7 +113,7 @@ tt_content {
         }
     }
 
-    vitec_downloadcard < lib.contentElement
+    vitec_downloadcard < lib.contentElementWithHeader
     vitec_downloadcard {
         fields {
             content {
@@ -104,7 +125,7 @@ tt_content {
         }
     }
 
-    vitec_downloadcardcollection < lib.contentElement
+    vitec_downloadcardcollection < lib.contentElementWithHeader
     vitec_downloadcardcollection {
         fields {
             content {
@@ -116,7 +137,7 @@ tt_content {
         }
     }
 
-    vitec_datasheets < lib.contentElement
+    vitec_datasheets < lib.contentElementWithHeader
     vitec_datasheets {
         fields {
             content {
@@ -127,6 +148,18 @@ tt_content {
             }
         }
     }
+
+    vitec_eventlist =< lib.contentElementWithHeader
+    vitec_eventlist {
+        fields {
+            content {
+                fields {
+                    eventlist = USER
+                    eventlist.userFunc = Evomedien\Vitec\UserFunc\EventlistJsonRenderer->render
+                }
+            }
+        }
+    }
 }
 
 # Include container (layout) rendering definitions