doxa-simple-map, cities-internet, all-people-groups, upg-neighbor,
prayer-walk-planner and demo-map compile useMapInstance in, so 0c1a91cd reaches
them only through a rebuild. check:shared-reached reports all five that use it as
carrying the guard.
cities-internet builds its map directly and never calls ensureEngineCSS, so it gets
its own guard: onMounted stops before new gl.Map if the pane unmounted during its
awaits. Removing the element 150, 400 or 900 ms after mount did not throw on either
build, so this is defensive, not a measured fix.
d6496bd87d