feat(xmldsig): URI dereference for Reference elements

[polaz]

Summary

Implement same-document URI resolution for XMLDSig <Reference> elements:

• Empty URI ("") → entire document without comments
• Bare-name #id → element subtree by ID attribute
• #xpointer(/) → document root with comments
• #xpointer(id('...')) → element subtree by ID (XPointer form)
• ID attribute registration (ID, Id, id + custom names)

Technical Details

• UriReferenceResolver builds a HashMap<&str, Node> index on construction for O(1) lookups
• NodeSet type tracks included/excluded nodes with subtree exclusion support
• TransformData enum (NodeSet / Binary) for the transform pipeline
• Integration tested: URI dereference → NodeSet → C14N canonicalization end-to-end

Test Plan

• 19 unit tests covering all URI forms, error cases, edge cases (duplicate IDs, custom attrs, SAML-like docs)
• 11 integration tests verifying URI → NodeSet → C14N pipeline produces correct canonical output
• Includes SAML-specific scenarios (assertion subtree, enveloped signature exclusion)