Add Collection Hover Pile effect for stacked cards

[georgeolaru]

Summary

Add a Pile theme-inspired hover effect for stacked Cards Collection blocks. Activated via the u-collection-hover-pile body class (controlled by Style Manager).

What this does

• At rest: Only the image is visible; title and meta are hidden (opacity: 0)
• On hover: Image dims via NB's Overlay Filter system, content fades in with directional slides:
  • Title: fade + slight upward slide (centered)
  • First meta: slides down from top edge
  • Last meta: slides up from bottom edge
• Content overlay covers the full card, making the entire card clickable via the media link
• Inner container uses flex-direction: column; justify-content: space-between to distribute meta/title

Editor support

• Scoped editor rules for content layout in stacked cards
• Works with NB's display: grid !important editor layout

Scope

• Only applies to stacked card layouts (nb-supernova--card-layout-stacked)
• Only on devices with hover capability (@media (hover: hover))
• Excludes single-column (hero) blocks

Files

• src/scss/utility/_collection-hover-pile.scss (new) — hover effect CSS
• src/scss/utility.scss — imports the new file
• dist/css/utility.css, dist/css/utility-rtl.css — compiled output