Embedded schema for the data-side configuration of an Attribute.

Captures the structural contract: type, default value, required flag, import-source mapping, relation config, and validation rules.

Fields

• type — the data type identifier (e.g. "binary", "integer", "relation")
• required — whether the attribute must have a value
• has_default / default — whether a default value exists and its shape
• source_column — which source column this maps to during import
• relation_config — flexible map for relation settings (target_type, target_set_id, matching_rules, etc.)
• validations — list of validation rule maps
• source — data source configuration map

Consumers may store additional ad-hoc keys at the ASA level; those are merged on top by AttrEngine.Cascade at resolution time.