feat: multi-listener with configurable proxy chaining

Each listener binds to its own port with an independent chain.
The "pool" keyword in a chain appends a random alive proxy from
the shared pool; multiple pool entries = multiple hops.

  :1080 -> Tor only (0 pool hops)
  :1081 -> Tor + 1 pool proxy
  :1082 -> Tor + 2 pool proxies

Shared resources (ProxyPool, Tor, metrics, semaphore, API) are
reused across listeners. FirstHopPool is shared per unique first
hop. Backward compatible: old listen/chain format still works.

docs/CHEATSHEET.md

@@ -38,6 +38,24 @@ Volumes: `./src` (ro), `./config/s5p.yaml` (ro), `~/.cache/s5p` → `/data` (poo
 cp config/example.yaml config/s5p.yaml     # create live config (gitignored)
 ```
 
+## Multi-Listener (config)
+
+```yaml
+listeners:
+  - listen: 0.0.0.0:1080
+    chain:
+      - socks5://127.0.0.1:9050             # Tor only
+  - listen: 0.0.0.0:1081
+    chain:
+      - socks5://127.0.0.1:9050
+      - pool                                 # Tor + 1 pool hop
+  - listen: 0.0.0.0:1082
+    chain:
+      - socks5://127.0.0.1:9050
+      - pool                                 # Tor + 2 pool hops
+      - pool
+```
+
 ## Performance Tuning (config)
 
 ```yaml