Add a mega menu with per-item featured content to the primary navigation (develop)

[mlaetitia]

Related to Linear T51ENG-2081 (no GitHub issue exists for this request).

Proposed changes

• Revive the osi theme's dormant mega menu: full-bleed dropdown panels for top-level items carrying the megamenu class, rendered by OSI_Megamenu_Walker with a section header row (eyebrow + tagline from the menu item's Description field).
• Header turns white with a page dim behind while a panel is open; the transparent-until-scroll behavior is kept.
• Keyboard, touch, and ARIA support; mobile submenu toggle accessibility fixes.
• New partner-editable featured content card per dropdown: two ACF fields on primary-menu items (a page/post picker and a heading), rendered as a fourth panel column with image, title, and excerpt. Items with no selection keep the plain three-column panel.
• Field registration and resolution live in inc/megamenu-featured.php. The feature degrades cleanly if ACF is deactivated, only publicly-viewable non-password-protected posts render, and the card image is lazy-loaded so it never consumes the page's fetchpriority=high slot in the header.

Why are these changes being made?

• The partner asked for a CNCF-style mega menu for the new seven-section navigation structure, including a featured area per dropdown that they can update themselves without code changes (confirmed in the Linear thread). Both the menu structure and the featured content are edited in Appearance > Menus, so the regular menu-building flow covers setup end to end.

Testing instructions

• Assign a menu to the Primary location with top-level items carrying the megamenu CSS class and nested sub-items.
• At >= 1200px width, hover or focus a top-level item: the panel opens full-bleed, the header turns white, the page dims; Escape closes it; tab focus and touch behave the same.
• In Appearance > Menus, expand a top-level item, pick a page or post under "Featured item", optionally change "Featured heading", and save. The panel now shows a fourth-column card (image, title, excerpt) linking to the selection.
• Clear the field and the panel returns to three columns. Select a draft, private, or password-protected post: no card renders.
• Below 1200px the accordion behavior is unchanged and no featured card appears.

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