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

[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

Setup: on the Menus screen (Appearance > Menus), open Screen Options (top right) and enable "CSS Classes" and "Description" under advanced menu properties - both fields are hidden by default.

Mega menu behavior:

• Give a top-level menu item (with nested sub-items) the megamenu CSS class and save. At >= 1200px width, hover or focus it: the panel opens full-bleed, the header turns white, the page dims; Escape closes it; tab focus and touch behave the same.
• The item's Description field renders as the panel tagline under the section eyebrow; leave it empty and only the eyebrow shows.
• Items WITHOUT the megamenu class keep the classic narrow dropdown - the feature is opt-in per item, so an existing menu renders unchanged after deploy.
• In the expanded menu item, pick a page or post under "Featured item", optionally change "Featured heading", save: the panel gains a fourth-column card (image, title, excerpt) linking to the selection. Clear the field and the panel returns to three columns. A draft, private, or password-protected selection renders no card.
• Deactivate ACF: the site must not fatal; panels render without cards and selections survive reactivation.

Shared surfaces (apply to any menu, including the current production menu):

• Below 1200px: the mobile accordion opens/closes correctly, toggle buttons announce their state (aria-expanded), and the hamburger still works.
• Placeholder items (URL "#") inside dropdowns render as non-clickable labels and are skipped when tabbing.
• Top-level items show the new green underline hover treatment (intended design change).

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