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_shared.php

@@ -27,6 +27,87 @@ use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
         ],
     ]);
 
+    // -------------------------------------------------------------------------
+    // Shared parent-level setting: column gap scale (1–5) for the whole grid.
+    // Concerns the container itself (not a single column); maps to a spacing
+    // token in the React frontend. Rendered in the "Grid" palette on top of the
+    // Appearance tab, above the per-column rows.
+    // -------------------------------------------------------------------------
+    ExtensionManagementUtility::addTCAcolumns('tt_content', [
+        'tx_vitec_gap' => [
+            'label' => $l . 'gap.label',
+            'config' => [
+                'type' => 'select',
+                'renderType' => 'selectSingle',
+                'items' => [
+                    ['label' => $l . 'gap.option.1', 'value' => '1'],
+                    ['label' => $l . 'gap.option.2', 'value' => '2'],
+                    ['label' => $l . 'gap.option.3', 'value' => '3'],
+                    ['label' => $l . 'gap.option.4', 'value' => '4'],
+                    ['label' => $l . 'gap.option.5', 'value' => '5'],
+                ],
+                'default' => '3',
+            ],
+        ],
+    ]);
+
+    // Parent-element palette: settings that concern the whole grid, not a column.
+    $GLOBALS['TCA']['tt_content']['palettes']['vitec_grid'] = [
+        'showitem' => 'tx_vitec_gap',
+    ];
+
+    // -------------------------------------------------------------------------
+    // Shared per-column flex alignment fields for the X-column containers.
+    // Positional: col1 = first column, col2 = second, ... (max 4 columns).
+    // "Align"   -> CSS align-items      (cross-axis alignment of the column's items)
+    // "Justify" -> CSS justify-content  (main-axis distribution within the column)
+    // Reused across vitec_cols_50_50 / 33_66 / 66_33 / 33_33_33 / 25_25_25_25.
+    // -------------------------------------------------------------------------
+    $alignItems = [
+        ['label' => $l . 'flex.align.flex_start', 'value' => 'flex-start'],
+        ['label' => $l . 'flex.align.center',     'value' => 'center'],
+        ['label' => $l . 'flex.align.flex_end',   'value' => 'flex-end'],
+        ['label' => $l . 'flex.align.stretch',    'value' => 'stretch'],
+    ];
+    $justifyItems = [
+        ['label' => $l . 'flex.justify.flex_start',    'value' => 'flex-start'],
+        ['label' => $l . 'flex.justify.center',        'value' => 'center'],
+        ['label' => $l . 'flex.justify.space_between', 'value' => 'space-between'],
+        ['label' => $l . 'flex.justify.space_around',  'value' => 'space-around'],
+        ['label' => $l . 'flex.justify.space_evenly',  'value' => 'space-evenly'],
+    ];
+
+    $columnFields = [];
+    for ($i = 1; $i <= 4; $i++) {
+        $columnFields['tx_vitec_col' . $i . '_align'] = [
+            'label' => $l . 'flex.align.label',
+            'config' => [
+                'type' => 'select',
+                'renderType' => 'selectSingle',
+                'items' => $alignItems,
+                'default' => 'stretch',
+            ],
+        ];
+        $columnFields['tx_vitec_col' . $i . '_justify'] = [
+            'label' => $l . 'flex.justify.label',
+            'config' => [
+                'type' => 'select',
+                'renderType' => 'selectSingle',
+                'items' => $justifyItems,
+                'default' => 'flex-start',
+            ],
+        ];
+    }
+    ExtensionManagementUtility::addTCAcolumns('tt_content', $columnFields);
+
+    // One palette per column: Align + Justify on a single row. The visible row
+    // label is overridden per container in its showitem (--palette--;<colLabel>;<palette>).
+    for ($i = 1; $i <= 4; $i++) {
+        $GLOBALS['TCA']['tt_content']['palettes']['vitec_col' . $i . '_flex'] = [
+            'showitem' => 'tx_vitec_col' . $i . '_align, tx_vitec_col' . $i . '_justify',
+        ];
+    }
+
     // Register VITEC wizard group header
     $GLOBALS['TYPO3_CONF_VARS']['BE']['defaultPageTSconfig'] = ($GLOBALS['TYPO3_CONF_VARS']['BE']['defaultPageTSconfig'] ?? '') . "\n" . <<<'TSCONFIG'
 mod.wizards.newContentElement.wizardItems.vitec {