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/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+    description: 'Uid of the "Megamenu" content element to publish on every page as `megaMenu`. 0 = do not publish at page level.'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