Function validate_element_structure 

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

Validate element declaration structural constraints

Top-level elements:

• Must have name attribute
• Must NOT have ref, minOccurs, maxOccurs, or form attributes

Local elements:

• Must have exactly one of name OR ref
• If ref is present, type/default/fixed/nillable/block/final are prohibited