Sizing prefixes become `@sizing=ItemSize::...`, and the sites whose weight was
reactive now carry a memo of `ItemSize` instead of one of `f32`. `node_ref=`
and `test_id=` become `@node_ref=` and `@test_id=`. Adds tests that a memo
behind `@sizing` relays a kind change to layout and that a tag takes both other
slots at once, and updates the guides.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017W8YfdzDmZcR8A76jRjvZY