Host APT repository at stable URL

Parent: #123

Host an APT repository at a stable, project-controlled URL so operators can run `echo "deb https://… stable main" | sudo tee /etc/apt/sources.list.d/fallout.list`.

## Definition of done

- Hosting decision recorded in the issue: GitHub Pages + `reprepro`/`aptly` vs Cloudsmith free tier vs self-hosted. (Default recommendation: GitHub Pages + `reprepro` — zero external cost, fits existing infra. Reconsider if `arm64` bandwidth becomes a problem.)
- Repo URL is stable and announced in `README.md` install instructions.
- `Release` + `Release.gpg` + `InRelease` present, signed by __L3__'s key.
- Suites: `stable` only for now (no `unstable` — out of scope).
- Components: `main`.
- Architectures: `amd64`, `arm64`.
- `apt-get update` against the source produces no warnings on the baseline distros.