feat(dom): add PHP 8.5 DOM compliance foundation

[Guikingone]

Summary

This draft checkpoint implements the PHP 8.5 DOM/libxml foundation and the
current DOM behavior surface against the frozen php-src 8.5.8 contract.

The branch currently includes:

• a source-locked, statically linked elephc-dom bridge for libxml2 2.15.3
  and bundled Lexbor 2.7.0;
• generated PHP 8.5.8 reflection/operation manifests and compiler-side
  declarations, lowering, wrapper materialization, ownership, and structured
  exception transport;
• legacy and modern document/node APIs, serialization, parsing, validation,
  XInclude, C14N, selectors, collections, token lists, namespaces, iterators,
  stream-wrapper integration, XPath callbacks, and native wrapper lifecycle;
• DTD named-node maps, entities, notations, tree mutation, DOMXPath and
  Dom\XPath, including exact dynamic argument contracts and callback arrays;
• runtime-class dispatch for native methods and virtual properties selected
  through mixed or heterogeneous wrapper unions, with feature detection that
  links every latent DOM branch.

The normative specification is locked at
docs/specs/php-dom-compliance.md. Detailed progress and reproducible gates
are maintained in .plans/php-dom-compliance.md.

Tracks #622.

Checkpoint status

This PR is intentionally a draft and is not yet ready to merge.

• current published checkpoint: ccde8612b1
• 603 frozen operation keys
• 564 explicit bridge routes
• 39 unimplemented SimpleXML/interoperability keys remaining:
  • 27 functions or methods
  • 12 SimpleXML object handlers
• the frozen DOM/libxml/SimpleXML PHPT ledgers remain pending final zero-exclusion replay
• final macOS ARM64, Linux ARM64, and Linux x86_64 evidence remains open
• final same-commit LOCK consensus from GLM 5.2, Kimi K2.7, and MiniMax M3
  remains open

The draft will be updated in place as those gates close.

Current checkpoint validation

• cargo test --lib ir_lower::tests::internal_extensions
  • 19 passed
• cargo test --test codegen_tests mixed_dispatch_with_dom_candidates_links_required_runtime
  • passed
• focused XPath dynamic-type, variadic mutation, coercion-order, and heap-debug regressions
  • passed
• cargo check --tests
  • passed
• exact CLI reproduction from the previous cross-suite linker failure
  • compiled and ran with Row:1:Ada
• assembly-comment alignment checks
  • passed
• git diff --check
  • passed

The replacement complete CI run for ccde8612b1 is active. Local Linux Docker
execution remains unavailable on the current macOS host, so the complete
supported-target matrix will be provided by CI before this draft is marked ready.