Function install_mdbook_mermaid 

fn install_mdbook_mermaid(
    cache: &Path,
    local_bin: &Path,
    version: &str,
) -> Result<()>

Installs mdbook-mermaid via cargo install if the cached version is stale.

The binary is placed at ~/.cache/mdbook-mermaid/bin/mdbook-mermaid and symlinked into ~/.local/bin/. A .version file records the installed version.