feat(xslt): implement standalone XSLT engine

[polaz]

Summary

• add a standalone safe-Rust XSLT 1.0 engine with XPath, EXSLT, serialization, resolver, clock, and typed budget contracts
• provide the bounded XSLT capability boundary needed for XML-security policy integration while replacing the remaining quick-xml paths with shared bounded XML input handling
• vendor the complete pinned libxslt oracle corpus and safe DOM/XPath foundations, with standards-backed strict behavior and explicit compatibility cases
• add backend, encoding, no-std, CI, release, documentation, and reviewer fixture-scope support required by the complete feature
• make XPath sum() deterministic in document order, enforce complete oracle-output comparisons, and bound cloned CDATA metadata
• honor XInclude text encoding precedence and reserve terminating-message growth before allocation

Validation

• cargo nextest run --workspace --all-features --no-fail-fast (3089 passed)
• cargo nextest run -p xml-sec-xslt --test libxslt_oracle --no-fail-fast (26 passed)
• cargo test --doc --workspace --all-features (15 passed)
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo build --workspace --all-features
• alloc-only host and wasm32-unknown-unknown checks
• support-crate version guard fixture and shellcheck
• 30 repeated Linux runs of the previously intermittent oracle case

Closes #141

Summary by CodeRabbit

• New Features
  • Added XML byte parsing with strict encoding detection, transcoding, and configurable size limits.
  • Added an XSLT 1.0 engine with XML, HTML, and text output, resource budgets, and caller-controlled external resource access.
  • Added configurable namespace-binding limits and caller-controlled XInclude and clock access during transformations.
• Bug Fixes
  • Improved certificate revocation checks and made XPath lang() comparisons case-insensitive.
  • XML parsing and transformation operations now apply consistent resource limits.
• Documentation
  • Expanded guidance on XML capabilities, backends, interoperability, and specifications.