feat: add stale proxy expiry based on last_seen TTL

Evict proxies not returned by sources for >3 refresh cycles and not
currently alive. Cleans up proxies removed upstream faster than waiting
for max_fails consecutive health test failures.

tests/test_pool.py

@@ -153,6 +153,54 @@ class TestProxyPoolWeight:
         pool.report_failure(hop)  # should not raise
 
 
+class TestProxyPoolStaleExpiry:
+    """Test stale proxy eviction."""
+
+    def test_stale_dead_proxy_evicted(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+
+        cfg = ProxyPoolConfig(sources=[], refresh=300)
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        # stale + dead: should be evicted
+        stale = ChainHop(proto="socks5", host="10.0.0.1", port=1080)
+        pool._proxies["socks5://10.0.0.1:1080"] = ProxyEntry(
+            hop=stale, alive=False, last_seen=now - 1200,
+        )
+        # fresh + dead: should survive (recently seen, might recover)
+        fresh_dead = ChainHop(proto="socks5", host="10.0.0.2", port=1080)
+        pool._proxies["socks5://10.0.0.2:1080"] = ProxyEntry(
+            hop=fresh_dead, alive=False, last_seen=now,
+        )
+
+        with patch.object(pool, "_test_proxy", new_callable=AsyncMock, return_value=False):
+            asyncio.run(pool._run_health_tests())
+
+        assert "socks5://10.0.0.1:1080" not in pool._proxies
+        assert "socks5://10.0.0.2:1080" in pool._proxies
+
+    def test_stale_alive_proxy_kept(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+
+        cfg = ProxyPoolConfig(sources=[], refresh=300)
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        # stale but alive: should survive (still passing health tests)
+        hop = ChainHop(proto="socks5", host="10.0.0.1", port=1080)
+        pool._proxies["socks5://10.0.0.1:1080"] = ProxyEntry(
+            hop=hop, alive=True, last_seen=now - 1200, last_ok=now,
+        )
+
+        with patch.object(pool, "_test_proxy", new_callable=AsyncMock, return_value=True):
+            asyncio.run(pool._run_health_tests())
+
+        assert "socks5://10.0.0.1:1080" in pool._proxies
+
+
 class TestProxyPoolFetchFile:
     """Test file source parsing."""