Setup C++ Infra in GiGL

[mkolodner-sc]

Scope of work done

C++ Infrastructure for GiGL

This PR adds the foundational C++ infrastructure to GiGL — everything needed to write, build, lint, and test C++ code alongside the existing Python codebase.

What's included

• Build tooling — scripts/build_cpp_extensions.py auto-discovers and builds pybind11 C++ extensions under gigl/csrc/. The C++ build is wired into make unit_test_py and make install_dev_deps so it happens automatically without a separate step.

• Formatting and linting — .clang-format and .clang-tidy configs establish the C++ style conventions (naming, formatting, static analysis checks). make format_cpp and make lint_cpp run these, and both are included in the standard CI targets.

• Dependency installation — requirements/install_cpp_deps.sh handles installing clang-format, clang-tidy, and cmake on both Mac and Linux, including the PATH setup needed on Mac where Homebrew intentionally doesn't override Apple's built-in clang.

• Compile commands generation — scripts/generate_compile_commands.py generates build/compile_commands.json so clang-tidy can resolve include paths correctly.

• Shared build constants — Compiler flags and source paths live in gigl/cpp_build_constants.py as a single source of truth used by both the build and lint scripts.

• Documentation — docs/cpp_style_guide.md documents the formatting and linting rules and how to run them.

• Unit test scaffolding — tests/unit/cpp/ with a CMakeLists and a smoke test to verify the infrastructure works end to end.

Where is the documentation for this feature?: N/A

Did you add automated tests or write a test plan?

Updated Changelog.md? NO

Ready for code review?: NO

[mkolodner-sc]

Removed the custom streaming helper — subprocess.run without a stdout argument already streams output to the terminal in real time (it just inherits the parent's stdout). The old approach using stdout=PIPE was actually worse since piped subprocesses buffer their output more aggressively. We also get free failure detection via check=True, which the old function didn't provide (it returned the exit code but never checked it).