v0.6.4 — MCP structured output + Dockerfile fix

Fixed

• Dockerfile: Rust 1.93 (was 1.87), add build-essential/cmake/clang for crypto deps
• Dockerfile: --no-default-features to skip BoringSSL/QUIC for faster Docker builds

Changed

• MCP tools submit, login, validate now return structured_content alongside text
• MCP benchmark tool reports error count per URL instead of silently dropping failures
• CHANGELOG entries for 0.6.1–0.6.4