build: add clang ci

[egolearner]

Greptile Summary

This PR extends the CI matrix in .github/workflows/main.yml to add a Clang build job (linux-x64-clang) alongside the existing GCC-default jobs, enabling clang-compatibility checks on Linux x64.

• Adds a new matrix entry linux-x64-clang (runs on ubuntu-24.04) with a compiler: clang property.
• Adds a compiler: default property to the three pre-existing matrix entries to make the matrix schema consistent.
• Adds a conditional "Install Clang" step (if: matrix.compiler == 'clang') that installs clang via apt-get and exports CC=clang / CXX=clang++ into $GITHUB_ENV, making the compiler available to all downstream steps (CMake build, C++ tests, C++ examples).
• The environment-variable approach ($GITHUB_ENV) is the correct GitHub Actions mechanism for propagating values across steps, and both the Build from source (scikit-build-core/CMake) and Run C++ Examples (standalone CMake) steps will automatically pick up CC/CXX.
• Minor concern: clang is installed without a pinned version, unlike every other tool in the workflow (cmake, pybind11, ninja, etc.), which could lead to silent version drift.

Confidence Score: 4/5

• This PR is safe to merge; it only adds a new CI job and does not touch any production code.
• The change is purely additive (a new CI matrix entry), is structurally sound, and uses the correct GitHub Actions patterns. The only concern is the unpinned clang version, which is a best-practice issue rather than a correctness bug.
• No files require special attention beyond the minor version-pinning note in .github/workflows/main.yml.

Important Files Changed

Filename	Overview
.github/workflows/main.yml	Adds a new linux-x64-clang matrix entry and a conditional "Install Clang" step. The approach is correct and well-structured, with one minor concern: the clang package is not version-pinned, unlike other dependencies in the workflow.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Push / PR / workflow_dispatch] --> B[lint job\nCode Quality Checks\nubuntu-24.04]
    B --> C{Matrix: build-and-test}
    C --> D[macos-arm64\nmacos-15\ncompiler: default]
    C --> E[linux-arm64\nubuntu-24.04-arm\ncompiler: default]
    C --> F[linux-x64\nubuntu-24.04\ncompiler: default]
    C --> G[linux-x64-clang ✨ NEW\nubuntu-24.04\ncompiler: clang]

    G --> H[Install Clang\napt-get install clang\nCC=clang / CXX=clang++]
    H --> I[Build from source\npip install via CMake]
    I --> J[Run C++ Tests]
    J --> K[Run Python Tests]
    K --> L[Run C++ Examples]

    F --> M[Build from source\npip install via CMake\ndefault GCC]
    M --> N[Run C++ Tests]
    N --> O[Run Python Tests]
    O --> P[Run C++ Examples]

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_{Last reviewed commit: f7312ee}

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