Wagtail: add HTML signal (dom-only misses response-first detection)

[danielserranoh]

Wagtail matches only via dom selectors ([data-block-key] + media rendition paths). Response-first / non-headless consumers don't evaluate dom, so Wagtail goes undetected even when the StreamField markup is present in the HTML.

This adds an html pattern for the StreamField block key — the same signal as the existing dom [data-block-key] rule, but statically matchable:

• data-block-key="[a-z0-9]{5}"

Note: like the existing dom rule, this only appears on pages rendering StreamField blocks.

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Test websites:

• https://wagtail.org/
• https://torchbox.com/
• https://www.mozilla.org/
• https://www.oxfam.org.uk/

Assisted by Claude Code.

[danielserranoh]

Closing this one. Per the discussion in #352: html is deprecated and slower, and HTTP Archive runs Wappalyzer with DOM detection — where the existing [data-block-key] dom rule already covers exactly this signal, so the addition is redundant here. It was aimed at response-first consumers that can't evaluate dom; I'll keep it on my side instead.

Thanks for the review.