Add opt-in legacy DOM reads and regression coverage

[jdalton]

Change

Add opt-in legacy DOM reads, collection filtering, and traversal fallbacks. Mode changes clear query plans; document changes preserve the flag. Modern mode keeps direct reads.

Updated to current master. Disabled, enabled, required, and defined helpers now use the legacy accessors. The three previous expected failures are passing tests. The shared host fixture refreshes live collections after DOM mutations. The merged relative-selector fix uses legacy parent traversal for sibling arguments, with unit and WPT regressions for both sibling combinators.

Validation

• 449 Node tests pass, with two inherited expected failures and five skips.
• 45 WPT pages pass, including native comparisons using the shared legacy host fixture.
• Type, lint, formatting, generated docs, and 22 packed-package tests pass locally.
• Coverage passes increased thresholds: statements 79.78%, branches 70.49%, functions 80.31%, lines 78.27%. Engine coverage comes from WPT; adapter coverage comes from Node.

This does not transpile JavaScript, polyfill the runtime, or claim that historical browsers were tested. Separate property-read and query-routing optimizations remain outside this PR.

Minimums are now statements 79%, branches 70%, functions 80%, and lines 78%. Added WPT regressions for scoped duplicate IDs, fragments, comment-skipping traversal, raw URLs, namespaced attributes, and form IDs.