Submit the radiation trefoil to simple-icons

### Problem
The `built with Fallout` badge carries its logo as an inline base64 data URI, because shields.io has no Fallout icon. The result is a single line of markdown about 640 characters long. It is awkward to copy by hand and noisy in a README diff.

### Outcome
The radiation trefoil is an accepted [simple-icons](https://github.com/simple-icons/simple-icons) icon, so the badge shortens to `?logo=fallout&logoColor=F5C800`. The data-URI form stays documented in `docs/badge.md` as the fallback for older shields.io caches.

### Acceptance criteria
- [ ] Fallout meets the simple-icons inclusion requirements (they only accept brands above a popularity bar — see their [contributing guide](https://github.com/simple-icons/simple-icons/blob/develop/CONTRIBUTING.md))
- [ ] Single-path monochrome SVG prepared to their spec, on their 24x24 grid
- [ ] Icon merged and shipped in a published simple-icons release
- [ ] `docs/badge.md`, `README.md`, and `CONTRIBUTING.md` switched to `?logo=fallout`, with the data URI kept as a documented fallback

### Notes
Blocked, not actionable yet. Fallout does not meet the popularity requirement today. Filed so the option is not forgotten.
The current data-URI badge and the trefoil SVG source both live in `docs/badge.md`.