♻️ Upgrade C++ infra & fix clang‑tidy issues

[kammce]

This PR introduces a comprehensive refactor of the C++ infrastructure for
async_context. The changes include:

• Migrated the project to CMake 4.0 and adopted libhal‑cmake‑util for
  build configuration, simplifying library creation and test integration.
• Replaced the legacy CMake options (BUILD_UNIT_TESTS, BUILD_BENCHMARKS,
  etc.) with the new libhal conventions (libhal_project_init,
  libhal_add_library, etc.).
• Updated the build system to enable C++23, removed legacy compiler flags
  (-fexceptions, -fno‑rtti) that are no longer required with the new
  tooling.
• Added a lightweight compile_commands_clangd_resolver.py script and
  VS Code settings to improve clang‑d diagnostics.
• Cleaned up the .clang-tidy configuration, removing unused checks and
  adding a header‑filter.
• Fixed several clang‑tidy warnings:
  • async_context.cppm – added missing [[nodiscard]], suppressed
    false‑positive analyzer checks, and removed an unnecessary block_by_io
    call.
  • Updated test helpers (basic_context.test.cpp, basics.test.cpp,
    etc.) to use [[maybe_unused]] where appropriate.
• Removed the old test harness (tests/main.test.cpp) and replaced it with
  per‑test main() functions that drive each suite.
• Updated the README to reflect the new build workflow and removed
  references to legacy configuration.

The net effect is a cleaner, more maintainable build system that aligns
with the current libhal ecosystem and resolves all clang‑tidy
complaints. This brings the project closer to a stable 1.0 release.