Add <xref> element validations per SPS 1.10 specification#1136

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copilot/create-validations-for-xref-element

Commits on Mar 19, 2026

Initial plan
Copilot
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Add xref validation rules per SPS 1.10: rid/ref-type presence, ref-type values, bibr presence, rid-id correspondence, transcript xref, aff self-closing

Copilot
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robertatakenaka
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