Keep hidden headers (header_layout 100) out of the JSON

Applied on all three render paths: stdWrap.if rule on
lib.contentElementWithHeader (before the plugin copies), the same rule in
the container element definitions, and a HiddenHeaderProcessor behind the
nb content-blocks data processor. headerLayout itself stays in the payload
so the front end can tell why the header is empty.

packages/vitec/Configuration/Sets/Vitecset/setup.typoscript

@@ -17,6 +17,34 @@ config {
 
 @import 'EXT:headless/Configuration/TypoScript/ContentElement/*.typoscript'
 
+# =============================================================================
+# header_layout 100 = "Hidden" (decision 2026-09-04): such headers stay OUT of
+# the JSON - header, subheader and headerLink render empty. headerLayout (100)
+# stays in the payload so the front end could still tell why. Placed BEFORE
+# the `< lib.contentElementWithHeader` copies below, which snapshot this lib.
+# The content-blocks `data` path gets the same rule via HiddenHeaderProcessor.
+# =============================================================================
+
+lib.contentElementWithHeader.fields.content.fields {
+    header.stdWrap.if {
+        value = 100
+        equals.field = header_layout
+        negate = 1
+    }
+    subheader.stdWrap.if {
+        value = 100
+        equals.field = header_layout
+        negate = 1
+    }
+    headerLink.stdWrap.if {
+        value = 100
+        equals.field = header_layout
+        negate = 1
+    }
+}
+
+lib.contentBlock.fields.data.dataProcessing.20 = Evomedien\Vitec\DataProcessing\HiddenHeaderProcessor
+
 # =============================================================================
 # Headless list-plugin JSON renderers (TYPO3 v14: each plugin is its own CType)
 #