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.

TASKS.md

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+# s5p -- Tasks
+
+## Current
+
+- [x] Scaffold project structure
+- [x] Implement SOCKS5 server
+- [x] Implement protocol handshakes (SOCKS5, SOCKS4/4a, HTTP CONNECT)
+- [x] Implement chain builder
+- [x] CLI and config loading
+- [x] Unit tests (config, proto)
+- [x] Documentation
+- [ ] Smoke test with Tor
+
+## Next
+
+- [ ] Integration tests with mock proxy server
+- [ ] SOCKS5 server-side authentication
+- [ ] Tor control port integration