v0.6.0

Changed

• Breaking: Citation.reference (String) renamed to Citation.refs (Vec<String>) to support multi-citation clusters (e.g., [smith2023; jones2024])
• Breaking: ExtensionMark::citation() and citation_with_page() now emit "refs" (array) instead of "ref" (string)
• Deserialization accepts both old "ref" (string) and new "refs" (array) for backward compatibility

Added

• Citation::multi() constructor for multi-reference citations
• Citation::first_ref() and Citation::refs() accessors
• ExtensionMark::multi_citation() convenience constructor
• ExtensionMark::get_string_array_attribute() for array-typed attributes
• ExtensionMark::get_citation_refs() helper supporting both "refs" and legacy "ref" keys
• ExtensionMark::normalize_citation_attrs() to migrate "ref" → "refs" in-place
• ExtensionBlock::get_string_array_attribute() for parity with ExtensionMark
• Backward-compatibility conformance tests for singular "ref" deserialization
• Multi-reference citation roundtrip tests