Create design roadmap for rust-binary-router-library

[leynos]

Summary

• formulate a roadmap for the design at docs/rust-binary-router-library-design.md
• reformat all documents using mdformat
• fix remaining markdownlint violations

Testing

• cargo fmt
• cargo clippy -- -D warnings
• cargo test
• npx markdownlint-cli2 "**/*.md"
• nixie docs/rust-binary-router-library-design.md

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https://chatgpt.com/codex/tasks/task_e_6849604e14748322aa47ea7926c0c756

Summary by Sourcery

Provide a detailed design roadmap for the rust-binary-router library and standardize documentation formatting by applying mdformat and resolving markdownlint violations.

New Features:

• Add a structured design roadmap document (docs/roadmap.md) outlining the core library architecture, middleware, extractors, and future enhancements

Enhancements:

• Reformat all markdown files using mdformat for consistent styling
• Fix remaining markdownlint violations across project documentation
• Update AGENTS.md with refined code style and documentation guidelines

Documentation:

• Add AGENTS.md updates and the design roadmap summary to the docs folder

Tests:

• Add markdownlint checks in CI for lint enforcement