Replace YAML-specific functions with generic ones accepting unmarshaler/marshaler
function parameters. This enables future support for JSON and other formats
while maintaining current YAML behavior.
- UnmarshalValidateYAML -> UnmarshalValidate(unmarshalFunc)
- UnmarshalValidateYAMLXSync -> UnmarshalValidateXSync(unmarshalFunc)
- SaveJSON -> SaveFile(marshalFunc)
- LoadJSONIfExist -> LoadFileIfExist(unmarshalFunc)
- Add UnmarshalValidateReader for reader-based decoding
Testing: all 12 staged test files updated to use new API