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.

README.md

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+# s5p
+
+SOCKS5 proxy server with Tor and proxy-chain support. Routes connections
+through configurable chains of SOCKS4, SOCKS5, and HTTP CONNECT proxies.
+
+## Features
+
+- SOCKS5 server (RFC 1928)
+- Proxy chaining: tunnel through multiple hops in sequence
+- Supported hop protocols: SOCKS5, SOCKS4/4a, HTTP CONNECT
+- Per-hop authentication (username/password)
+- DNS leak prevention (domain names forwarded to proxies, never resolved locally)
+- Tor integration (Tor is just another SOCKS5 hop)
+- Pure Python, asyncio-based, minimal dependencies
+
+## Quick Start
+
+```bash
+# Install
+cd ~/git/s5p
+python -m venv .venv && source .venv/bin/activate
+pip install -e .
+
+# Run with Tor
+s5p -C socks5://127.0.0.1:9050
+
+# Run with a chain: Tor -> external proxy
+s5p -C socks5://127.0.0.1:9050,socks5://proxy:1080
+
+# Run with config file
+s5p -c config/example.yaml
+
+# Test it
+curl --proxy socks5h://127.0.0.1:1080 https://check.torproject.org/api/ip
+```
+
+## Configuration
+
+```yaml
+listen: 127.0.0.1:1080
+timeout: 10
+
+chain:
+  - socks5://127.0.0.1:9050          # Tor
+  - socks5://user:pass@proxy:1080    # exit-side proxy
+  - http://proxy2:8080               # HTTP CONNECT proxy
+```
+
+## CLI Reference
+
+```
+s5p [-c FILE] [-l [HOST:]PORT] [-C URL[,URL,...]] [-t SEC] [-v|-q]
+
+Options:
+  -c, --config FILE        YAML config file
+  -l, --listen [HOST:]PORT Listen address (default: 127.0.0.1:1080)
+  -C, --chain URL[,URL]    Comma-separated proxy chain
+  -t, --timeout SEC        Per-hop timeout (default: 10)
+  -v, --verbose            Debug logging
+  -q, --quiet              Errors only
+  -V, --version            Show version
+```
+
+## How Chaining Works
+
+```
+Client -> s5p -> Hop1 -> Hop2 -> ... -> HopN -> Destination
+```
+
+s5p connects to Hop1 via TCP, negotiates the hop protocol (SOCKS5/4/HTTP),
+then over that tunnel negotiates with Hop2, and so on. The final hop connects
+to the actual destination. Each hop only sees its immediate neighbors.