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/TCA/Overrides/tt_content_vitec_container.php

@@ -32,8 +32,7 @@ use TYPO3\CMS\Core\Utility\GeneralUtility;
 
     $GLOBALS['TCA']['tt_content']['types']['vitec_container']['showitem'] =
         '--palette--;;general,
-         header;LLL:EXT:vitec/Resources/Private/Language/locallang_containers.xlf:section.heading,
-         subheader;LLL:EXT:vitec/Resources/Private/Language/locallang_containers.xlf:section.subline,
+         --palette--;;headers,
          tx_vitec_bg_variant,
          pi_flexform;LLL:EXT:vitec/Resources/Private/Language/locallang_containers.xlf:container.flexform.label,
          --div--;LLL:EXT:frontend/Resources/Private/Language/locallang_ttc.xlf:tabs.appearance,