feat(c14n): xml:* attribute inheritance + C14N 1.1 xml:base fixup for document subsets

[polaz]

Summary

Implement W3C C14N spec requirements for document subsets:

1. xml: attribute inheritance (G001)* — When Inclusive C14N processes a document subset and an element's parent is NOT in the node set, inheritable xml:* attributes from ancestor elements are propagated to the topmost included element per W3C C14N §2.4:

   • C14N 1.0: xml:lang, xml:space, xml:base
   • C14N 1.1: adds xml:id to the above
   • Exclusive C14N omits this inheritance entirely per Exc-C14N §3

2. C14N 1.1 mode enabled (P1-007) — Previously returned UnsupportedAlgorithm. Now routes to inclusive renderer with xml:id propagation and xml:base URI resolution

3. xml:base URI fixup (G004) — C14N 1.1 resolves relative xml:base URIs in document subsets via RFC 3986 §5. The effective URI is absolute only if the ancestor chain includes a scheme-bearing base; otherwise it may remain relative. Both inherited and own xml:base values are resolved against the effective ancestor chain

Test Plan

• 9 unit tests for xml:* inheritance (serialize.rs) — including no-inheritance-past-included-ancestor
• 5 unit tests for C14N 1.1 mode (mod.rs)
• 27 unit tests for RFC 3986 URI resolver (xml_base.rs) — including schemeless base, file: scheme, empty xml:base
• 9 integration tests for C14N 1.1 + xml:base fixup (c14n_integration.rs)
• All 148 tests pass, clippy clean