Add Display Text field type for static read-only product page information

Problem

No field type exists for showing read-only information (ship dates, disclaimers, batch numbers) on the product page without creating an editable input.

Solution

New display input type:

• Admin: textarea for display content, CSS class option, visibility settings
• Frontend: renders as <div class="ppom-display-text"> with wp_kses_post() sanitization
• Does NOT submit data to cart (no <input> element in template)

Files Affected

• New: classes/inputs/input.display.php — input class
• New: templates/frontend/inputs/display.php — frontend template
• inc/arrays.php — register in ppom_array_all_inputs() core array
• classes/ppom.class.php — skip in has_unique_datanames() check

Acceptance Criteria

• "Display Text" appears in field type picker
• Renders as read-only text on product page
• Does NOT appear in cart, checkout, or order details
• HTML content properly sanitized via wp_kses_post()

Priority: Low — single user request, nice-to-have
Regression risk: Isolated — new field type

Start by comparing existing input classes in classes/inputs and frontend templates in templates/frontend/inputs, then trace registration through inc/arrays.php and the has_unique_datanames() check in classes/ppom.class.php. Add the new display type in the named files and verify the picker, sanitized read-only rendering, and absence from cart, checkout, and order details.