Replace catalog previews with per-project SVG thumbnails

[kkd16]

Summary

• replace live catalog iframe previews with lazy-loaded static SVG images
• require and validate a self-contained 16:10 public/thumbnail.svg in source and build output
• add one stock Replace me with an SVG placeholder to the template and all 47 existing projects
• update project guidance and catalog metadata generation

Verification

• node --test scripts/validation.test.mjs
• node scripts/discover-projects.mjs: 47 conforming, 0 rejected
• node scripts/build-site.mjs: 47 published, 0 rejected after frozen install, lint, build, and output validation
• browser smoke: 47 cards, 47 thumbnail images, 0 iframes
• git diff --check

Merge note

This is intentionally a repo-wide catalog migration, so it touches the catalog infrastructure plus every project thumbnail. The single-project auto-merge policy will reject its scope; please merge it manually.

[github-actions]

🚫 Out of scope. changes outside a project folder are never auto-merged (e.g. "AGENTS.md")

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. "AGENTS.md")

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