Function ensure_dependency 

pub fn ensure_dependency(
    cargo_toml: &Path,
    name: &str,
    spec: &str,
) -> Result<bool, CodegenError>

Ensure <name> = <spec> is listed under [dependencies] in a Cargo.toml.

Ok(true) = added, Ok(false) = already there (idempotent), Err(_) = the file has no [dependencies] section to add it to, and the caller should report it rather than invent one.

The generator needs this because code scaffolded into a plugin crate usually needs a dependency the plugin doesn’t have yet — a REST permission class in plugins/blog/ doesn’t compile until plugins/blog/Cargo.toml depends on umbral-rest. Writing the file and leaving the crate unable to build it is a generator that produced a broken project.

String surgery, not a TOML rewrite: comments, ordering and formatting of the existing manifest all survive.