feat: add weighted proxy selection based on recency

Replace uniform random.choice with random.choices weighted by last_ok
recency. Proxies tested successfully more recently get higher selection
probability (weight = 1/(1 + age/300)), decaying over ~5 minutes.

tests/test_pool.py

@@ -72,6 +72,51 @@ class TestProxyPoolGet:
         assert result.host == "1.2.3.4"
 
 
+class TestProxyPoolWeight:
+    """Test weighted proxy selection."""
+
+    def test_recent_proxy_preferred(self):
+        import asyncio
+        from collections import Counter
+
+        cfg = ProxyPoolConfig(sources=[])
+        pool = ProxyPool(cfg, [], timeout=10.0)
+
+        now = time.time()
+        fresh = ChainHop(proto="socks5", host="10.0.0.1", port=1080)
+        stale = ChainHop(proto="socks5", host="10.0.0.2", port=1080)
+        pool._proxies["socks5://10.0.0.1:1080"] = ProxyEntry(
+            hop=fresh, alive=True, last_ok=now,
+        )
+        pool._proxies["socks5://10.0.0.2:1080"] = ProxyEntry(
+            hop=stale, alive=True, last_ok=now - 3600,
+        )
+        pool._rebuild_alive()
+
+        counts: Counter[str] = Counter()
+        for _ in range(1000):
+            hop = asyncio.run(pool.get())
+            counts[hop.host] += 1
+
+        assert counts["10.0.0.1"] > counts["10.0.0.2"] * 3
+
+    def test_weight_values(self):
+        hop = ChainHop(proto="socks5", host="1.2.3.4", port=1080)
+        now = 1000.0
+
+        # just tested
+        entry = ProxyEntry(hop=hop, last_ok=now)
+        assert ProxyPool._weight(entry, now) == pytest.approx(1.0)
+
+        # 5 minutes ago
+        entry.last_ok = now - 300
+        assert ProxyPool._weight(entry, now) == pytest.approx(0.5)
+
+        # never tested
+        entry.last_ok = 0
+        assert ProxyPool._weight(entry, now) == pytest.approx(0.01)
+
+
 class TestProxyPoolFetchFile:
     """Test file source parsing."""