feat: initial SOCKS5 proxy with chain support

Asyncio-based SOCKS5 server that tunnels connections through
configurable chains of SOCKS5, SOCKS4/4a, and HTTP CONNECT proxies.
Tor integration via standard SOCKS5 hop.

docs/CHEATSHEET.md

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+# s5p -- Cheatsheet
+
+## CLI
+
+```
+s5p                                         # direct, listen :1080
+s5p -C socks5://127.0.0.1:9050             # through Tor
+s5p -C socks5://tor:9050,http://px:8080    # Tor + HTTP proxy
+s5p -c config/example.yaml                  # from config file
+s5p -l 0.0.0.0:9999                         # custom listen address
+s5p -t 30                                   # 30s per-hop timeout
+s5p -v                                      # debug logging
+s5p -q                                      # errors only
+```
+
+## Proxy URLs
+
+```
+socks5://host:port
+socks5://user:pass@host:port
+socks4://host:port
+http://host:port
+http://user:pass@host:port
+```
+
+## Testing
+
+```bash
+# Check exit IP
+curl -x socks5h://127.0.0.1:1080 https://httpbin.org/ip
+
+# Verbose curl
+curl -v -x socks5h://127.0.0.1:1080 https://example.com
+
+# With timeout
+curl --max-time 30 -x socks5h://127.0.0.1:1080 https://example.com
+```
+
+## Troubleshooting
+
+| Symptom | Check |
+|---------|-------|
+| Connection refused | Is Tor running? `ss -tlnp \| grep 9050` |
+| Timeout | Increase `-t`, check proxy reachability |
+| DNS leak | Use `socks5h://` (not `socks5://`) in client |
+| Auth failed | Verify credentials in proxy URL |