TLS 1.2 fallback for HTTPS servers without TLS 1.3 #218

Some HTTPS origins still require TLS 1.2. For example, on 2026-05-17, OpenSSL negotiates TLSv1.2 with https://httpbin.org/get and the we TLS 1.3-only ClientHello receives a warning close_notify before ServerHello.\n\nRepro:\n\n cargo run -q -p we-e2e -- --url https://httpbin.org/get --out /tmp/we-httpbin.png\n\nCurrent result:\n\n render error: fetch https://httpbin.org/get: Network(Tls(Tls(AlertReceived(Alert { level: Warning, description: CloseNotify }))))\n\nOpenSSL confirmation:\n\n openssl s_client -connect httpbin.org:443 -servername httpbin.org -tls1_3 -groups X25519 -ciphersuites TLS_AES_128_GCM_SHA256 -alpn 'h2,http/1.1'\n\nreads only a close_notify and no peer certificate, while the same command without -tls1_3 negotiates TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256.\n\nAcceptance criteria:\n\n- Add a TLS 1.2 client fallback sufficient for common HTTPS servers such as httpbin.org.\n- Preserve TLS 1.3 as the preferred path.\n- Add an e2e scenario or targeted network test covering a TLS 1.2-only HTTPS origin.\n- cargo fmt --all, cargo clippy --workspace -- -D warnings, and cargo test --workspace pass.