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.

tests/test_config.py

@@ -2,7 +2,14 @@
 
 import pytest
 
-from s5p.config import ChainHop, Config, load_config, parse_api_proxies, parse_proxy_url
+from s5p.config import (
+    ChainHop,
+    Config,
+    ListenerConfig,
+    load_config,
+    parse_api_proxies,
+    parse_proxy_url,
+)
 
 
 class TestParseProxyUrl:
@@ -212,3 +219,143 @@ class TestConfig:
         assert c.proxy_pool.test_targets == [
             "www.google.com", "www.cloudflare.com", "www.amazon.com",
         ]
+
+
+class TestListenerConfig:
+    """Test multi-listener config parsing."""
+
+    def test_defaults(self):
+        lc = ListenerConfig()
+        assert lc.listen_host == "127.0.0.1"
+        assert lc.listen_port == 1080
+        assert lc.chain == []
+        assert lc.pool_hops == 0
+
+    def test_listeners_from_yaml(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "listeners:\n"
+            "  - listen: 0.0.0.0:1080\n"
+            "    chain:\n"
+            "      - socks5://127.0.0.1:9050\n"
+            "  - listen: 0.0.0.0:1081\n"
+            "    chain:\n"
+            "      - socks5://127.0.0.1:9050\n"
+            "      - pool\n"
+            "  - listen: 0.0.0.0:1082\n"
+            "    chain:\n"
+            "      - socks5://127.0.0.1:9050\n"
+            "      - pool\n"
+            "      - pool\n"
+        )
+        c = load_config(cfg_file)
+        assert len(c.listeners) == 3
+
+        # listener 0: no pool hops
+        assert c.listeners[0].listen_host == "0.0.0.0"
+        assert c.listeners[0].listen_port == 1080
+        assert len(c.listeners[0].chain) == 1
+        assert c.listeners[0].pool_hops == 0
+
+        # listener 1: 1 pool hop
+        assert c.listeners[1].listen_port == 1081
+        assert len(c.listeners[1].chain) == 1
+        assert c.listeners[1].pool_hops == 1
+
+        # listener 2: 2 pool hops
+        assert c.listeners[2].listen_port == 1082
+        assert len(c.listeners[2].chain) == 1
+        assert c.listeners[2].pool_hops == 2
+
+    def test_pool_keyword_stripped_from_chain(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "listeners:\n"
+            "  - listen: 1080\n"
+            "    chain:\n"
+            "      - socks5://tor:9050\n"
+            "      - pool\n"
+            "      - pool\n"
+        )
+        c = load_config(cfg_file)
+        lc = c.listeners[0]
+        # only the real hop remains in chain
+        assert len(lc.chain) == 1
+        assert lc.chain[0].host == "tor"
+        assert lc.pool_hops == 2
+
+    def test_pool_keyword_case_insensitive(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "listeners:\n"
+            "  - listen: 1080\n"
+            "    chain:\n"
+            "      - Pool\n"
+        )
+        c = load_config(cfg_file)
+        assert c.listeners[0].pool_hops == 1
+        assert c.listeners[0].chain == []
+
+
+class TestListenerBackwardCompat:
+    """Test backward-compatible single listener from old format."""
+
+    def test_old_format_creates_single_listener(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "listen: 0.0.0.0:9999\n"
+            "chain:\n"
+            "  - socks5://127.0.0.1:9050\n"
+        )
+        c = load_config(cfg_file)
+        assert len(c.listeners) == 1
+        lc = c.listeners[0]
+        assert lc.listen_host == "0.0.0.0"
+        assert lc.listen_port == 9999
+        assert len(lc.chain) == 1
+        assert lc.pool_hops == 0
+
+    def test_empty_config_creates_single_listener(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text("")
+        c = load_config(cfg_file)
+        assert len(c.listeners) == 1
+        lc = c.listeners[0]
+        assert lc.listen_host == "127.0.0.1"
+        assert lc.listen_port == 1080
+
+
+class TestListenerPoolCompat:
+    """Test that proxy_pool + old format auto-sets pool_hops=1."""
+
+    def test_pool_auto_appends(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "listen: 0.0.0.0:1080\n"
+            "chain:\n"
+            "  - socks5://127.0.0.1:9050\n"
+            "proxy_pool:\n"
+            "  sources:\n"
+            "    - url: http://api:8081/proxies\n"
+        )
+        c = load_config(cfg_file)
+        assert len(c.listeners) == 1
+        lc = c.listeners[0]
+        assert lc.pool_hops == 1
+
+    def test_explicit_listeners_no_auto_append(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "listeners:\n"
+            "  - listen: 0.0.0.0:1080\n"
+            "    chain:\n"
+            "      - socks5://127.0.0.1:9050\n"
+            "proxy_pool:\n"
+            "  sources:\n"
+            "    - url: http://api:8081/proxies\n"
+        )
+        c = load_config(cfg_file)
+        assert len(c.listeners) == 1
+        lc = c.listeners[0]
+        # explicit listeners: no auto pool_hops
+        assert lc.pool_hops == 0