Add evo_megamenu_json extension and seed the megamenu

- new project-neutral extension evo_megamenu_json (Evomedien): a Megamenu
  plugin holding entries, columns, items, teaser cards and a featured slot
  as nested records; presentation settings travel with the payload so the
  front end reads behaviour instead of hard-coding it; published as
  page.10.fields.megaMenu, driven by the site settings megamenu.contentUid
  and megamenu.storagePid
- vitec:seed-megamenu fills one element from the live structure: 5 entries,
  26 columns, 95 items, 4 story cards; items without a page yet are flagged
  pending so the front end falls back to the column link
- vitec:debug-sets prints the resolved site set order
- read-only diagnostics: check_megamenu_schema.php, check_typoscript_templates.php
- docs: schema updates run via extension:setup (database:updateschema no
  longer exists in v14), and a JSON Accept header activates headless-mixed,
  which strips every set-added page field

packages/evo_megamenu_json/Configuration/Sets/Megamenu/config.yaml

@@ -0,0 +1,9 @@
+name: evomedien/megamenu
+label: EVO Megamenu (JSON)
+
+# headless has to be loaded before this set: it resets the whole page object
+# (`page < lib.headlessPage`), which would drop the page-level field added
+# here. Declaring the dependency keeps that order without the site
+# configuration having to care about the listing position.
+dependencies:
+  - friendsoftypo3/headless

packages/evo_megamenu_json/Configuration/Sets/Megamenu/settings.definitions.yaml

@@ -0,0 +1,19 @@
+categories:
+  megamenu:
+    label: 'Megamenu'
+    description: 'Which megamenu element is published at page level'
+
+settings:
+  megamenu.contentUid:
+    label: 'Megamenu element (uid)'
+    description: 'Uid of the "Megamenu" content element to publish on every page as `megaMenu`. 0 = do not publish at page level.'
+    category: megamenu
+    type: int
+    default: 0
+
+  megamenu.storagePid:
+    label: 'Megamenu folder (pid)'
+    description: 'Alternative to the uid above: the first megamenu element found on this folder is published. Ignored when an element uid is set.'
+    category: megamenu
+    type: int
+    default: 0

packages/evo_megamenu_json/Configuration/Sets/Megamenu/setup.typoscript

@@ -0,0 +1,41 @@
+# =============================================================================
+# EVO Megamenu JSON
+#
+# Two wiring points, one renderer:
+#   1. the plugin element renders itself, so the menu can be previewed on the
+#      page it sits on (content.megamenu),
+#   2. the site header needs the menu on every page - set `megamenu.contentUid`
+#      (or `megamenu.storagePid`) and it is published at page level as
+#      `megaMenu`.
+#
+# Two things are deliberate here:
+#
+#   * No TypoScript condition around the page-level field. A condition that
+#     cannot be evaluated swallows every include that follows it and silently
+#     disables the TypoScript of all later sets. The renderer returns an empty
+#     value when nothing is configured - same result, no risk.
+#
+#   * The JSON key is written out literally. Since TYPO3 v12 the TypoScript
+#     parser substitutes constants only on the VALUE side, so a `{$...}` in
+#     the object path produces no field at all (found the hard way,
+#     2026-09-11). A project that needs another key copies these five lines
+#     into its own set.
+#
+# The USER function returns a JSON string; the headless JSON content object
+# decodes it, so the menu arrives as a real object, not an escaped string.
+# =============================================================================
+
+tt_content.evomegamenujson_megamenu =< lib.contentElement
+tt_content.evomegamenujson_megamenu {
+    fields {
+        megamenu = USER
+        megamenu.userFunc = Evomedien\EvoMegamenuJson\UserFunc\MegamenuJsonRenderer->render
+    }
+}
+
+page.10.fields.megaMenu = USER
+page.10.fields.megaMenu {
+    userFunc = Evomedien\EvoMegamenuJson\UserFunc\MegamenuJsonRenderer->render
+    contentUid = {$megamenu.contentUid}
+    storagePid = {$megamenu.storagePid}
+}