The shell forwarded tk, tokenUrl, locale and treeBase to each nested map,
but not the engine. So <doxa-maps-spa provider="maplibre"> still loaded
mapbox-gl and Mapbox styles. On a keyless host, that is the embed failing.
The shell now forwards `provider` in each child's profile-config, validated
to the two known engines. That fixed the simple map tabs, but not the
composites: they mount their inner maps without passing the provider on.
Measured with one tab per clean page: Big Picture made 19 api.mapbox.com
requests, Regions 17, UPG-100 15, Simple Research 14. So when a host asks
for an engine, the shell also sets window.MAP_PROVIDER (only if the page
hasn't). That is rung 3 of every map's own ladder, so it reaches maps
nested at any depth. After it, all four composites make 0 Mapbox requests.
check:gallery-embed:maplibre runs the cross-origin gate with a keyless
provider and fails on any api.mapbox.com request, attributed to the tab
that was open. Before this change, js mode failed with 20 URLs. It has one
named, printed exemption: upg-100-list.vue pins a token-free mapbox-gl.css.
That belongs to the upg100 plugin and is routed there.
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