Function write_plugin_caixa 

pub fn write_plugin_caixa(
    spec: &EscribaPluginSpec,
    artifacts: &CaixaArtifacts,
    out: &Path,
) -> Result<PathBuf, ForgeError>

Materialize a forged plugin caixa into <out>/<name>/…. Creates the directory tree and writes all four artifacts. Overwrites existing files (so a re-forge is idempotent).