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/templates/backend-preview.html

@@ -0,0 +1,19 @@
+<html xmlns:f="http://typo3.org/ns/TYPO3/CMS/Fluid/ViewHelpers" data-namespace-typo3-fluid="true">
+<f:layout name="Preview"/>
+<f:section name="Content">
+
+<f:asset.css identifier="vitec-backend-preview" href="EXT:vitec/Resources/Public/Css/backend-preview.css"/>
+
+<div class="vitec-preview">
+    <div class="vitec-preview__thumb-placeholder">▦</div>
+    <div class="vitec-preview__body">
+        <div class="vitec-preview__label">VITEC · Columns</div>
+        <h3 class="vitec-preview__headline">Two-column layout — {data.layout}</h3>
+    </div>
+    <div class="vitec-preview__settings">
+        <span class="vitec-badge">{data.layout}</span>
+    </div>
+</div>
+
+</f:section>
+</html>