Get Zone Adjacency

Which zone may follow which, for one park's zones.

Drives the custom-route picker: night N+1 offers only zones reachable from night N's selection, so the route the user builds is bookable rather than being rejected at creation.

The constrained flag is what the client must branch on, NOT the emptiness of adjacency:

• constrained=false → this provider's graph has not been crawled, or the provider has no sequence constraint at all. Free-pick. (For a constrained provider that also means custom routes are refused at creation — the wizard should offer curated routes only.)
• constrained=true with an empty entry for a zone → that zone genuinely leads nowhere; it is not a "no data" case.

Collapsing those two would either block routes in parks that permit everything, or offer routes the provider will refuse.