host: popover geometry (card placement, label, constants) is the host's

Pure placement of a trusted local card under a button of a site tab, kept inside the tab, flipped
above when short, with the caret on the button; moved from the X app with its seven tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/waybetter-host/src/lib.rs

@@ -65,6 +65,7 @@ pub mod layout;
 pub mod media;
 pub mod modes;
 pub mod plugins;
+pub mod popover;
 pub mod policy;
 pub mod profiles;
 pub mod safe;