Implement html-page-lang-matches-default rule (ucwvc8)

## ACT Rule
- **ACT Rule ID:** [ucwvc8](https://www.w3.org/WAI/standards-guidelines/act/rules/ucwvc8/)
- **Rule Name:** HTML page language subtag matches default language
- **WCAG SC:** 3.1.1 Language of Page (Level A)

## What it checks
This rule checks that the `lang` attribute of the `` element matches the default language of the page content.

## Feasibility
**DOM-only feasible.** Involves reading the `lang` attribute on `` and comparing against page content language.

## Notes
- Related to issue #350 (ACT rules triage)
- Companion to the existing `html-has-lang` and `html-lang-valid` rules
- May require a language detection heuristic for the page content

Start by examining the existing html-has-lang and html-lang-valid rules, then read the linked ACT rule ucwvc8. Determine how page-content language should be detected and compared with the html lang attribute. Done means the new rule reliably reports whether the page language subtag matches the default content language.

Written by the indexing model from the issue text.