feat: add fast warm start with deferred full health test

On warm start (state has alive proxies), only quick-test the
previously-alive subset before serving. Full health test runs in
background. Cold start behavior unchanged (test all before serving).
Reduces startup blocking from minutes to seconds on warm restarts.

tests/test_pool.py

@@ -153,6 +153,38 @@ class TestProxyPoolWeight:
         pool.report_failure(hop)  # should not raise
 
 
+class TestProxyPoolHealthTests:
+    """Test selective health testing."""
+
+    def test_selective_keys(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
+
+        cfg = ProxyPoolConfig(sources=[])
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        hop_a = ChainHop(proto="socks5", host="10.0.0.1", port=1080)
+        hop_b = ChainHop(proto="socks5", host="10.0.0.2", port=1080)
+        pool._proxies["socks5://10.0.0.1:1080"] = ProxyEntry(
+            hop=hop_a, alive=False, last_seen=now,
+        )
+        pool._proxies["socks5://10.0.0.2:1080"] = ProxyEntry(
+            hop=hop_b, alive=False, last_seen=now,
+        )
+
+        # only test proxy A
+        with patch.object(pool, "_test_proxy", new_callable=AsyncMock, return_value=True) as mock:
+            asyncio.run(pool._run_health_tests(keys=["socks5://10.0.0.1:1080"]))
+            # should only have been called for proxy A
+            assert mock.call_count == 1
+            assert mock.call_args[0][0] == "socks5://10.0.0.1:1080"
+
+        assert pool._proxies["socks5://10.0.0.1:1080"].alive is True
+        # proxy B untouched
+        assert pool._proxies["socks5://10.0.0.2:1080"].alive is False
+
+
 class TestProxyPoolStaleExpiry:
     """Test stale proxy eviction."""