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.

src/s5p/pool.py

@@ -69,9 +69,24 @@ class ProxyPool:
     async def start(self) -> None:
         """Load state, fetch sources, run initial health test, start loops."""
         self._load_state()
+        warm_keys = list(self._alive_keys)
         await self._fetch_all_sources()
-        await self._run_health_tests()
-        self._save_state()
+
+        if warm_keys:
+            # warm start: quick-test previously-alive proxies first
+            valid_keys = [k for k in warm_keys if k in self._proxies]
+            if valid_keys:
+                await self._run_health_tests(keys=valid_keys)
+                self._save_state()
+                self._tasks.append(asyncio.create_task(self._deferred_full_test()))
+            else:
+                await self._run_health_tests()
+                self._save_state()
+        else:
+            # cold start: test everything before serving
+            await self._run_health_tests()
+            self._save_state()
+
         self._tasks.append(asyncio.create_task(self._refresh_loop()))
         self._tasks.append(asyncio.create_task(self._health_loop()))
 
@@ -252,11 +267,23 @@ class ProxyPool:
             except OSError:
                 pass
 
-    async def _run_health_tests(self) -> None:
-        """Test all proxies with bounded concurrency."""
+    async def _run_health_tests(self, keys: list[str] | None = None) -> None:
+        """Test proxies with bounded concurrency.
+
+        Args:
+            keys: Subset of proxy keys to test. None tests all.
+        """
         if not self._proxies:
             return
 
+        target = (
+            [(k, self._proxies[k]) for k in keys if k in self._proxies]
+            if keys is not None
+            else list(self._proxies.items())
+        )
+        if not target:
+            return
+
         sem = asyncio.Semaphore(self._cfg.test_concurrency)
         results: dict[str, bool] = {}
 
@@ -267,7 +294,7 @@ class ProxyPool:
                 except Exception:
                     results[key] = False
 
-        tasks = [_test(k, e) for k, e in list(self._proxies.items())]
+        tasks = [_test(k, e) for k, e in target]
         await asyncio.gather(*tasks)
 
         total = len(results)
@@ -330,6 +357,11 @@ class ProxyPool:
 
     # -- background loops ----------------------------------------------------
 
+    async def _deferred_full_test(self) -> None:
+        """Run a full health test in background after warm start."""
+        await self._run_health_tests()
+        self._save_state()
+
     async def _refresh_loop(self) -> None:
         """Periodically re-fetch sources and merge."""
         while not self._stop.is_set():

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."""