VerticalTabsLayout, header off: on a phone a 44px bar (a real grid row) replaces the floating corner
circle, with the rail toggle at the inline start and a #phone-end slot at the end; on a desktop a
#rail-end slot sits at the bottom of the rail (it receives `collapsed`). Neither renders with a header
on, so header-on consumers and desktop layouts without a filler are unchanged. The --map-fab-* footprint
is no longer published; its readers fall back to their plain insets.
ShellLanguageMenu fills either slot: the switcher's languages plus the current one, a native select
under a globe face. A pick dispatches one cancelable, composed, bubbling doxa-language-request on the
host element (library/utils/languageRequest.js). A host that cancels keeps the language; otherwise the
menu emits 'switch' and writes ?lang= only on a page the tree generated.
check:shell-header (gates:pre, Vite SSR) asserts where the bar and the rail slot render, the event's
shape and count, cancel and no-cancel, the ?lang= rule and the language list; its self-test (gates:post)
refuses 11 planted defects. check:nav-reachable now presses the bar's toggle.
