Document why imperative DOM modules live outside React

[andimrob]

Problem

~40% of the JS by line count (xray.ts, confetti.ts, coinCollect.ts) creates/destroys DOM nodes imperatively, outside React's component tree. This isn't a bug — they're ephemeral overlays — but it's invisible to React DevTools and may confuse future contributors.

Recommendation

• Add a brief comment at the top of each file explaining why it's outside React (fire-and-forget overlay, no React state needed, avoids re-render coupling)
• If these ever need to compose with React state (e.g. "show confetti when a project card is clicked"), convert them to React portals at that point
• No immediate code changes needed

Priority

Low — documentation/clarity improvement only.