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.

config/example.yaml

@@ -50,6 +50,29 @@ chain:
 #   cookie_file: ""                # CookieAuthentication file path
 #   newnym_interval: 0             # periodic NEWNYM (seconds, 0 = manual only)
 
+# Multi-listener mode -- each listener gets its own address and chain.
+# The "pool" keyword in a chain appends a random alive proxy from the pool.
+# Multiple "pool" entries = multiple pool hops (deeper chaining).
+#
+# listeners:
+#   - listen: 0.0.0.0:1080
+#     chain:
+#       - socks5://127.0.0.1:9050       # Tor only (no pool hops)
+#
+#   - listen: 0.0.0.0:1081
+#     chain:
+#       - socks5://127.0.0.1:9050
+#       - pool                           # Tor + 1 random pool proxy
+#
+#   - listen: 0.0.0.0:1082
+#     chain:
+#       - socks5://127.0.0.1:9050
+#       - pool                           # Tor + 2 random pool proxies
+#       - pool
+#
+# When using "listeners:", the top-level "listen" and "chain" keys are ignored.
+# If "listeners:" is absent, the old format is used (single listener).
+
 # Legacy proxy source (still supported, auto-converts to proxy_pool):
 # proxy_source:
 #   url: http://10.200.1.250:8081/proxies