Function validate_attribute_structure 

pub fn validate_attribute_structure(
    attrs: &AttributeMap,
    name_table: &NameTable,
    ctx: &ValidationContext,
) -> SchemaResult<()>

Validate attribute declaration structural constraints

Top-level attributes:

• Must have name attribute
• Must NOT have ref, use, or form attributes

Local attributes:

• Must have exactly one of name OR ref
• If ref is present, type/form are prohibited (src-attribute.3.2)
• default and fixed ARE allowed on refs (they set the attribute use’s value constraint)