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.

PROJECT.md

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+# s5p -- Project
+
+## Purpose
+
+A lightweight SOCKS5 proxy server that chains connections through Tor and/or
+arbitrary proxy hops (SOCKS4, SOCKS5, HTTP CONNECT).
+
+## Motivation
+
+Existing solutions (`proxychains-ng`) rely on `LD_PRELOAD` hacks, only work
+on Linux, and intercept at the library level. s5p is a proper SOCKS5 server
+that any application can use natively -- no injection required.
+
+## Architecture
+
+```
+                  TCP          tunnel        tunnel
+Client -------> s5p -------> Hop 1 -------> Hop 2 -------> Target
+        SOCKS5       proto1         proto2         protoN
+```
+
+- **server.py** -- asyncio SOCKS5 server, chain builder, bidirectional relay
+- **proto.py** -- protocol handshake implementations (SOCKS5, SOCKS4/4a, HTTP CONNECT)
+- **config.py** -- YAML config loading, proxy URL parsing
+- **cli.py** -- argparse CLI, logging setup
+
+## Dependencies
+
+| Package | Purpose |
+|---------|---------|
+| pyyaml  | Config file parsing |
+
+All other functionality uses Python stdlib (`asyncio`, `socket`, `struct`).
+
+## Design Decisions
+
+- **No LD_PRELOAD** -- clean SOCKS5 server, works with any client
+- **asyncio** -- single-threaded event loop, efficient for I/O-bound proxying
+- **Domain passthrough** -- never resolve DNS locally to prevent leaks
+- **Tor as a hop** -- no special Tor handling; it's just `socks5://127.0.0.1:9050`