sequenceDiagram
    participant User
    participant RunEnv as Test Environment<br/>(py313)
    participant PkgEnv as .pkg<br/>(Package Env)
    participant WheelEnv as .pkg-cpython313<br/>(wheel_build_env)
    participant TmpDir as .pkg/tmp/

    User->>RunEnv: tox run with package=sdist-wheel
    RunEnv->>PkgEnv: Request package
    
    Note over PkgEnv,WheelEnv: Registration Phase
    PkgEnv->>PkgEnv: Add "sdist-wheel" to package.py:80<br/>wheel_build_env check
    PkgEnv->>WheelEnv: Register wheel_build_env<br/>(same as wheel/editable)
    
    rect rgb(255, 244, 225)
        Note over PkgEnv: Phase 1: Build sdist
        PkgEnv->>PkgEnv: Install build-system.requires
        PkgEnv->>PkgEnv: Build sdist → my-pkg.tar.gz
    end
    
    rect rgb(240, 225, 255)
        Note over PkgEnv,TmpDir: Phase 2: Extract
        PkgEnv->>TmpDir: Extract to env_tmp_dir/sdist-extract/
        Note over TmpDir: Same location as cmd_builder.py:111
        TmpDir-->>PkgEnv: Extracted source path
    end
    
    rect rgb(225, 255, 240)
        Note over WheelEnv: Phase 3: Build wheel
        PkgEnv->>WheelEnv: Get from _wheel_build_envs[wheel_build_env]
        PkgEnv->>WheelEnv: Set root = extracted_path
        Note over WheelEnv: Root setter closes old backend,<br/>creates new frontend
        WheelEnv->>WheelEnv: Read pyproject.toml from extracted sdist
        WheelEnv->>WheelEnv: Install build deps<br/>(from extracted pyproject.toml)
        WheelEnv->>WheelEnv: Build wheel using existing logic<br/>(lines 283-300 in pyproject.py)
        WheelEnv-->>PkgEnv: Return wheel
    end
    
    rect rgb(245, 245, 245)
        Note over TmpDir: Phase 4: Cleanup
        Note over TmpDir: env_tmp_dir cleared automatically<br/>on next setup (api.py:350-356)
        Note over PkgEnv: No manual shutil.rmtree needed
    end
    
    PkgEnv-->>RunEnv: Install wheel
    RunEnv->>User: Run tests