Expand Shadowsocks runtime compatibility

deploy/railway-sing-box/README.md

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+# Railway sing-box Shadowsocks Test Node
+
+This is a minimal sing-box Shadowsocks server for testing Burrow against a real
+internet-hosted node on Railway.
+
+Railway public networking exposes raw TCP through TCP Proxy. That is enough for
+plain Shadowsocks TCP testing. It is not a full UDP validation environment, so
+use Burrow's local self-tests or a VPS for native UDP testing.
+
+## Deploy
+
+1. Create a new Railway service from this repo.
+2. Set the service root directory to:
+
+   ```text
+   deploy/railway-sing-box
+   ```
+
+3. Add Railway variables:
+
+   ```text
+   SS_PASSWORD=<strong random password>
+   SS_METHOD=chacha20-ietf-poly1305
+   SS_LISTEN_HOST=0.0.0.0
+   SS_LISTEN_PORT=8388
+   ```
+
+   `SS_METHOD`, `SS_LISTEN_HOST`, and `SS_LISTEN_PORT` are optional; those are
+   the defaults.
+
+4. Deploy the service.
+5. In the service settings, create a TCP Proxy with internal port `8388`.
+6. Railway will show an external proxy host and port, for example:
+
+   ```text
+   shuttle.proxy.rlwy.net:15140
+   ```
+
+## Burrow Test URI
+
+Build the Shadowsocks URI with:
+
+```sh
+uv run python - <<'PY'
+import base64
+import urllib.parse
+
+method = "chacha20-ietf-poly1305"
+password = "<SS_PASSWORD>"
+host = "<RAILWAY_TCP_PROXY_DOMAIN>"
+port = "<RAILWAY_TCP_PROXY_PORT>"
+name = "railway-sing-box-ss"
+
+userinfo = base64.urlsafe_b64encode(f"{method}:{password}".encode()).decode().rstrip("=")
+print(f"ss://{userinfo}@{host}:{port}#{urllib.parse.quote(name)}")
+PY
+```
+
+Use the generated `ss://` URI in a Burrow proxy subscription or local test
+payload.
+
+## Notes
+
+- Railway's TCP Proxy external port is assigned by Railway. Use that external
+  port in the client URI, not `8388`.
+- This is intentionally a plain Shadowsocks node. Once plain TCP works, add
+  extra protocol features in separate test deployments so failures stay easy to
+  isolate.