getCurrentLocation was calling isTravelAllowed per connection, re-querying
the character's discovered locations once per gated exit. Extract the
gating rule into a pure, synchronous WorldDiscoveryService.isRouteOpen so
the map filter can load the discovery set once and filter in memory, while
isTravelAllowed (TravelService's entry point) keeps its exact signature and
behaviour by delegating to the same rule. Also pins that discoversLocationKey
never reaches the client payload.
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