feat: Add mcr & update compat.imgui 1.93.0 - #536

- Add mcr (Modular C++23 HTTP client on libcurl) package descriptor
  as Form A module library with identity yizhinailong:mcr
- Include comprehensive documentation in
  .agents/docs/2026-10-16-add-mcr-plan.md covering source details,
  descriptor design, feature assessment, testing and risk analysis
- Add mcr to README examples with consumer test reference
- Create test suite in tests/examples/mcr validating module compilation,
  linking and behavior for version constants, status classifications,
  URL/header handling, JSON operations and response parsing
- Configure package descriptor with tag archive URLs and SHA-256 hashes
  for all three platforms (linux, macosx, windows)