Add tree-sitter HTML and CSS grammars for .html/.htm/.xhtml and .css
highlighting, and light up HTML's <script>->javascript / <style>->css
injections (the north-star injection consumer). Both crates export their query
constants over the tree-sitter-language shim (no overlay, ABI-fine). The only
reconciliation is two new capture entries (tag, attribute) in highlight.rs.
Framing approved through three review rounds.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
