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/columns/config.yaml

@@ -0,0 +1,91 @@
+name: vitec/columns
+group: vitec_custom_components
+prefixFields: true
+prefixType: vendor
+
+fields:
+  - 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: layout
+    type: Select
+    renderType: selectSingle
+    default: cols_50_50
+    items:
+      - label: '50 / 50'
+        value: cols_50_50
+      - label: '66 / 33'
+        value: cols_66_33
+      - label: '33 / 66'
+        value: cols_33_66
+
+  - identifier: items
+    type: Collection
+    label: Items
+    labelField: headline
+    fields:
+      - identifier: column
+        type: Select
+        renderType: selectSingle
+        default: left
+        items:
+          - label: Left column
+            value: left
+          - label: Right column
+            value: right
+      - identifier: item_type
+        type: Select
+        renderType: selectSingle
+        default: text
+        items:
+          - label: Text
+            value: text
+          - label: Image
+            value: image
+          - label: Video
+            value: video
+          - label: CTA / Button
+            value: cta
+      - identifier: headline
+        type: Text
+        max: 255
+      - identifier: text
+        type: Textarea
+        enableRichtext: true
+      - identifier: image
+        type: File
+        minitems: 0
+        maxitems: 1
+        allowed: common-image-types
+      - identifier: video
+        type: File
+        minitems: 0
+        maxitems: 1
+        allowed: mp4,webm,ogv,mov,m4v
+      - identifier: link
+        type: Link
+        allowedTypes:
+          - page
+          - url
+          - file
+          - email
+      - identifier: link_label
+        type: Text
+        max: 60