feat: replace HTTP health check with TLS handshake

Replace _http_check (HTTP GET to httpbin.org) with _tls_check that
performs a TLS handshake through the proxy chain. Multiple targets
(google, cloudflare, amazon) rotated round-robin eliminate the single
point of failure. Lighter, faster, harder to block than HTTP.

- Add test_targets config field (replaces test_url)
- Backward compat: legacy test_url extracts hostname automatically
- Add ssl.create_default_context() and round-robin index to ProxyPool
- Update docs (example.yaml, USAGE.md, CHEATSHEET.md)

tests/test_config.py

@@ -175,3 +175,40 @@ class TestConfig:
         cfg_file.write_text("listen: 1080\n")
         c = load_config(cfg_file)
         assert c.tor is None
+
+    def test_proxy_pool_test_targets(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "proxy_pool:\n"
+            "  sources: []\n"
+            "  test_targets:\n"
+            "    - host-a.example.com\n"
+            "    - host-b.example.com\n"
+        )
+        c = load_config(cfg_file)
+        assert c.proxy_pool is not None
+        assert c.proxy_pool.test_targets == ["host-a.example.com", "host-b.example.com"]
+        assert c.proxy_pool.test_url == ""
+
+    def test_proxy_pool_legacy_test_url(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "proxy_pool:\n"
+            "  sources: []\n"
+            "  test_url: http://httpbin.org/ip\n"
+        )
+        c = load_config(cfg_file)
+        assert c.proxy_pool is not None
+        assert c.proxy_pool.test_targets == ["httpbin.org"]
+
+    def test_proxy_pool_defaults(self, tmp_path):
+        cfg_file = tmp_path / "test.yaml"
+        cfg_file.write_text(
+            "proxy_pool:\n"
+            "  sources: []\n"
+        )
+        c = load_config(cfg_file)
+        assert c.proxy_pool is not None
+        assert c.proxy_pool.test_targets == [
+            "www.google.com", "www.cloudflare.com", "www.amazon.com",
+        ]