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/TypoScript/Headless/vitec_containers.typoscript

@@ -2,6 +2,12 @@
 # VITEC container content elements — self-contained JSON rendering
 # Children collected via Evomedien\Vitec\DataProcessing\ContainerChildrenProcessor
 # (exception-safe; returns [] on any error, never crashes the outer CE).
+#
+# Per-column flex settings (align/justify) live on the PARENT record
+# (tx_vitec_col{1..4}_align/justify) and are attached by the processor to each
+# column in `items` (items[].config.align/justify) — so they apply to ALL
+# children of that column. The whole-grid gap (tx_vitec_gap) is emitted here as
+# the parent-level `gap`.
 # =============================================================================
 
 tt_content.vitec_cols_50_50 = JSON
@@ -70,9 +76,27 @@ tt_content.vitec_cols_50_50 {
         header.field = header
         subheader = TEXT
         subheader.field = subheader
+        headerLayout = INT
+        headerLayout.field = header_layout
+        headerPosition = TEXT
+        headerPosition.field = header_position
+        headerLink = TEXT
+        headerLink {
+            htmlSpecialChars = 1
+            typolink {
+                parameter.field = header_link
+                returnLast = result
+            }
+        }
         tx_vitec_bg_variant = TEXT
         tx_vitec_bg_variant.field = tx_vitec_bg_variant
 
+        # Parent-level: column gap scale (1–5) for the whole grid.
+        gap = TEXT
+        gap.field = tx_vitec_gap
+
+        # Columns with their children + per-column flex (align/justify) in
+        # items[].config — assembled by ContainerChildrenProcessor.
         items = JSON
         items {
             dataProcessing {
@@ -83,18 +107,48 @@ tt_content.vitec_cols_50_50 {
     }
 }
 
-tt_content.vitec_cols_33_33_33 =< tt_content.vitec_cols_50_50
-tt_content.vitec_cols_25_25_25_25 =< tt_content.vitec_cols_50_50
-tt_content.vitec_cols_66_33 =< tt_content.vitec_cols_50_50
-tt_content.vitec_cols_33_66 =< tt_content.vitec_cols_50_50
+tt_content.vitec_cols_33_33_33 < tt_content.vitec_cols_50_50
+tt_content.vitec_cols_25_25_25_25 < tt_content.vitec_cols_50_50
+tt_content.vitec_cols_66_33 < tt_content.vitec_cols_50_50
+tt_content.vitec_cols_33_66 < tt_content.vitec_cols_50_50
 
 # -----------------------------------------------------------------------------
 # VITEC · Container — single-column container with a custom CSS class.
 # Inherits the full structure of the grid containers and adds the `cssClass`
 # field, read from the element's FlexForm (settings.cssClass).
+# Single column -> no grid gap.
 # -----------------------------------------------------------------------------
-tt_content.vitec_container =< tt_content.vitec_cols_50_50
+tt_content.vitec_container < tt_content.vitec_cols_50_50
+tt_content.vitec_container.fields.gap >
 tt_content.vitec_container.fields.cssClass = TEXT
 tt_content.vitec_container.fields.cssClass.data = flexform:pi_flexform:settings.cssClass
 tt_content.vitec_container.fields.cssClass.stdWrap.ifEmpty.cObject = TEXT
 tt_content.vitec_container.fields.cssClass.stdWrap.ifEmpty.cObject.value =
+
+# -----------------------------------------------------------------------------
+# VITEC · Cards Carousel — container whose children (typically vitec_card
+# elements) become the slides. Inherits the grid-container JSON structure
+# (`items` via ContainerChildrenProcessor) and adds carousel settings read
+# from the element's FlexForm. Single slide track -> no grid gap.
+# -----------------------------------------------------------------------------
+tt_content.vitec_cards_carousel < tt_content.vitec_cols_50_50
+tt_content.vitec_cards_carousel.fields.gap >
+tt_content.vitec_cards_carousel.fields.carousel = JSON
+tt_content.vitec_cards_carousel.fields.carousel {
+    fields {
+        slidesPerView = TEXT
+        slidesPerView.data = flexform:pi_flexform:settings.slidesperview
+        showArrows = TEXT
+        showArrows.data = flexform:pi_flexform:settings.showarrows
+        showIndicators = TEXT
+        showIndicators.data = flexform:pi_flexform:settings.showindicators
+        loop = TEXT
+        loop.data = flexform:pi_flexform:settings.loop
+        autoplay = TEXT
+        autoplay.data = flexform:pi_flexform:settings.autoplay
+        autoplayInterval = TEXT
+        autoplayInterval.data = flexform:pi_flexform:settings.autoplayinterval
+    }
+}
+tt_content.vitec_cards_carousel.fields.cssClass = TEXT
+tt_content.vitec_cards_carousel.fields.cssClass.data = flexform:pi_flexform:settings.cssClass