The Experimental maps' explainer lived in a "?" floating at the top centre of the app, which read
as a game piece rather than part of the map. The map already has a "?" control. This lets that
control answer from the HOST: a click dispatches doxa:help, bubbling and composed, so it crosses
every shadow root between the button and the page, and the app answers with the map's text.
library/composables/useHostedHelp.js is the one mechanism, for the HelpButton map control and for
the maps whose "?" belongs in their own bar. It also decides WHERE the button shows: only under a
host that answers (an ancestor, across shadow roots, carrying data-doxa-help), so the same bundle
embedded on a partner page shows no dead "?". HelpButton keeps its open-help emit, so
doxa-simple-map's own help popup is unchanged; hosted is opt-in.
