Complete DTD identity and reference validation semantics

[FlorianRappl]

Description

Implement full validation semantics for tokenized attribute types related to identity and references, including:

• ID uniqueness constraints
• IDREF and IDREFS referential checks
• ENTITY and ENTITIES declaration checks
• better integration with document-level ID lookup behavior

Background

User scenarios include:

• validating configuration or interchange XML with strict DTD rules
• catching invalid references during CI instead of runtime
• ensuring compatibility with systems that enforce XML validity constraints

Current validation covers many cases but still leaves important identity/reference constraints partially enforced.

Specification

Relevant specifications:

• W3C XML 1.0 (Fifth Edition)
• Validity Constraint: ID
• Validity Constraint: One ID per Element Type
• Validity Constraint: IDREF
• Validity Constraint: Entity Name
• Attribute declaration rules in section 3.3.1 and related constraints in section 3