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)
e78fc8dc3c