Function parse_includes 

pub fn parse_includes(
    includes_seq: &[Value],
    model: &mut ParsedModel,
) -> ForgeResult<()>

Parse _includes section from YAML (v4.0 cross-file references)

Expected format:

_includes:
  - file: "data_sources.yaml"
    as: "sources"
  - file: "pricing.yaml"
    as: "pricing"

Errors

Returns an error if an include entry is missing required file or as fields, or is not a valid mapping.