bin/railway now derives EXPECTED_DOMAINS from
showcase/scripts/railway-envs.generated.json instead of maintaining a
parallel Ruby hash. The TS railway-envs.ts is canonical; CI guards drift
via `emit-railway-envs-json.ts --check`. Adds a Minitest parity test that
boots Ruby and asserts its derived EXPECTED_DOMAINS matches the SSOT
JSON (public hosts only, env-id keys match SSOT envIds). Refs spec §3a.
[BLITZ:A6]
Jordan Ritter