Optimize catalog preview loading

[kkd16]

Summary

• replace eager blank iframe contexts with lightweight preview placeholders
• keep at most two visible app previews alive
• hand preview slots to the cards nearest the viewport center
• stagger the two starts with simple timers
• remove previews when they leave the viewport, are filtered out, or the tab is hidden
• reduce each preview render surface from 400% to 250%

Simplification review

• removed idle-callback abstraction, watchdogs, failure/retry state, and queue-state bookkeeping
• reduced the branch from 208 added lines to 92
• removed all comments introduced by this branch

Verification

• node --check assets/catalog.js
• git diff main --check
• node scripts/build-site.mjs --catalog-only (45 projects published, 0 rejected)
• headless Chromium smoke test: 45 cards, 45 placeholders, exactly 2 live iframes, 2 ready
• DevTools scroll trace across 15 positions: slots now switch to each incoming row while its thumbnails are entering the viewport and remain stable through the visible pass

Merge note

This intentionally updates the shared catalog shell (assets/catalog.js and assets/catalog.css), so it requires the requested manual admin merge rather than the project-only auto-merge path.

[github-actions]

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

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

[github-actions]

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

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