Stabilize catalog previews while scrolling

[kkd16]

Summary

• rank intersecting preview cards by distance to the viewport center
• recalculate handoff as intersection ratios change during scrolling
• keep the existing two-iframe limit from Optimize catalog preview loading #750

This fixes incoming previews loading late and disappearing shortly afterward because cards already above the viewport retained the two available slots.

Verification

• node --check assets/catalog.js
• git diff origin/main...HEAD --check
• node scripts/build-site.mjs --catalog-only (47 projects published, 0 rejected)
• DevTools trace across 15 scroll positions confirmed each incoming row receives both slots while entering the viewport and retains them through its visible pass

Scope

This branch starts from current origin/main after #750 and changes only assets/catalog.js (10 additions, 2 deletions). It requires manual admin merge because the project-only auto-merge gate intentionally rejects catalog-shell changes.

[github-actions]

🚫 Out of scope. changes outside a project folder are never auto-merged (e.g. "assets/catalog.js")

Change only one projects/<slug>/ folder, then push again. Run node scripts/verify-project.mjs <slug> first — it catches this locally.