Add build verification to CI workflow

[kashifpk]

Summary

Add a build verification job to the CI workflow to ensure the package builds correctly on every PR and push.

Implementation

Add a new job to .github/workflows/ci.yml:

build:
  name: Build Package
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4

    - name: Install uv
      uses: astral-sh/setup-uv@v4
      with:
        version: "latest"

    - name: Set up Python
      run: uv python install

    - name: Build package
      run: uv build

    - name: Upload build artifacts
      uses: actions/upload-artifact@v4
      with:
        name: dist
        path: dist/

Acceptance Criteria

• Build job runs on push to main and PRs
• Build artifacts are uploaded for inspection
• Build failure blocks PR merge