Split facet-html-dom from facet-html (issue #1529)

[fasterthanlime]

Summary

Splits the HTML DOM element types into a new facet-html-dom crate so they can properly use the #[facet(html::elements)] attribute namespace instead of falling back to #[facet(xml::elements)].

Fixes #1529

Changes

• New crate facet-html-dom: Contains all typed HTML5 element definitions (Html, Head, Body, Div, Span, etc.)
• DOM types now use canonical html:: attributes:
  • #[facet(html::elements)] for child element collections
  • #[facet(html::attribute)] for HTML attributes
  • #[facet(html::text)] for text content
• Updated facet-html: Now contains only the parser, serializer, and attribute grammar
• Updated all tests to import from facet_html_dom directly

Why the split?

Due to Rust limitation rust-lang/rust#52234, macro-exported attributes cannot be referenced by absolute path from within the same crate. By moving DOM types to a separate crate that depends on facet-html, they can properly use the html:: namespace.

Migration

Users should update imports:

// Before
use facet_html::elements::{Html, Div, P};

// After  
use facet_html_dom::{Html, Div, P};

Test plan

• All 2482 existing tests pass
• DOM types correctly use html:: attributes (129 occurrences, 0 xml:: references)