Function generate_codecs 

pub fn generate_codecs(
    root: &Path,
    types: &[BoundaryType],
    mode: CodegenMode,
) -> Result<CodecReport, CodegenError>

Generates (or, in CodegenMode::Check, verifies) the package’s codecs module src/<package>_codecs.gleam from its boundary types.

The codecs are derived from the types module alone — not the activity declarations — so this runs before the package’s manifest() is executed to extract the declarations: the author’s activities module references codecs.<type>_codec(), which must already compile for the extraction build to succeed. generate_activities emits the remaining plumbing (wrappers, worker, golden) once the declarations are known.

Errors

Returns a CodegenError for an unreadable gleam.toml, a write failure, or — in check mode — a missing or drifted codecs module.