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.

tests/test_proto.py

@@ -0,0 +1,22 @@
+"""Tests for protocol helpers."""
+
+from s5p.proto import Socks5AddrType, encode_address
+
+
+class TestEncodeAddress:
+    """Test SOCKS5 address encoding."""
+
+    def test_ipv4(self):
+        atyp, data = encode_address("127.0.0.1")
+        assert atyp == Socks5AddrType.IPV4
+        assert data == b"\x7f\x00\x00\x01"
+
+    def test_ipv6(self):
+        atyp, data = encode_address("::1")
+        assert atyp == Socks5AddrType.IPV6
+        assert len(data) == 16
+
+    def test_domain(self):
+        atyp, data = encode_address("example.com")
+        assert atyp == Socks5AddrType.DOMAIN
+        assert data == bytes([11]) + b"example.com"