feat: multi-Tor round-robin via tor_nodes config

New top-level tor_nodes list distributes traffic across multiple Tor
SOCKS proxies. First hop is replaced at connection time by round-robin
selection; health tests also rotate across all nodes. FirstHopPools
are created for each node when pool_size > 0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_pool.py

@@ -22,6 +22,38 @@ class TestProxyEntry:
         assert entry.tests == 0
 
 
+class TestEffectiveChain:
+    """Test chain_nodes round-robin in pool health tests."""
+
+    def test_no_nodes_returns_original(self):
+        cfg = ProxyPoolConfig(sources=[])
+        chain = [ChainHop(proto="socks5", host="10.0.0.1", port=9050)]
+        pool = ProxyPool(cfg, chain, timeout=10.0)
+        assert pool._effective_chain() == chain
+
+    def test_round_robin_across_nodes(self):
+        cfg = ProxyPoolConfig(sources=[])
+        chain = [ChainHop(proto="socks5", host="original", port=9050)]
+        nodes = [
+            ChainHop(proto="socks5", host="node-a", port=9050),
+            ChainHop(proto="socks5", host="node-b", port=9050),
+            ChainHop(proto="socks5", host="node-c", port=9050),
+        ]
+        pool = ProxyPool(cfg, chain, timeout=10.0, chain_nodes=nodes)
+
+        hosts = [pool._effective_chain()[0].host for _ in range(6)]
+        assert hosts == [
+            "node-a", "node-b", "node-c",
+            "node-a", "node-b", "node-c",
+        ]
+
+    def test_empty_chain_no_replacement(self):
+        cfg = ProxyPoolConfig(sources=[])
+        nodes = [ChainHop(proto="socks5", host="node-a", port=9050)]
+        pool = ProxyPool(cfg, [], timeout=10.0, chain_nodes=nodes)
+        assert pool._effective_chain() == []
+
+
 class TestProxyPoolMerge:
     """Test proxy deduplication and merge."""