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/USAGE.md

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+# s5p -- Usage
+
+## Basic Usage
+
+```bash
+# Direct proxy (no chain, just a SOCKS5 server)
+s5p
+
+# Through Tor
+s5p -C socks5://127.0.0.1:9050
+
+# Through Tor + another proxy
+s5p -C socks5://127.0.0.1:9050,socks5://proxy:1080
+
+# Custom listen address
+s5p -l 0.0.0.0:9999 -C socks5://127.0.0.1:9050
+
+# From config file
+s5p -c config/example.yaml
+
+# Debug mode
+s5p -v -C socks5://127.0.0.1:9050
+```
+
+## Config File
+
+```yaml
+listen: 127.0.0.1:1080
+timeout: 10
+log_level: info
+
+chain:
+  - socks5://127.0.0.1:9050
+  - http://user:pass@proxy:8080
+```
+
+## Proxy URL Format
+
+```
+protocol://[username:password@]host[:port]
+```
+
+| Protocol | Default Port | Auth Support |
+|----------|-------------|-------------|
+| socks5   | 1080        | username/password |
+| socks4   | 1080        | none |
+| http     | 8080        | Basic |
+
+## Testing the Proxy
+
+```bash
+# Check exit IP via Tor
+curl --proxy socks5h://127.0.0.1:1080 https://check.torproject.org/api/ip
+
+# Fetch a page
+curl --proxy socks5h://127.0.0.1:1080 https://example.com
+
+# Use with Firefox: set SOCKS5 proxy to 127.0.0.1:1080, enable remote DNS
+```
+
+Note: use `socks5h://` (not `socks5://`) with curl to send DNS through the proxy.
+
+## Chain Order
+
+Hops are traversed left-to-right:
+
+```
+-C hop1,hop2,hop3
+
+Client -> s5p -> hop1 -> hop2 -> hop3 -> Destination
+```
+
+Each hop only sees its immediate neighbors.