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/Resources/Public/Icons/item.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"><circle cx="3.2" cy="8" r="1.6" fill="#1D4ED8"/><path fill="#94A3B8" d="M6 6.4h8v1.5H6zM6 9.1h5.5v1.3H6z"/></svg>