CachedLayerRegistry

Registry of (dataType, serviceType) → CachedLayerOpener. The same key pair the 2.x cache surface uses for dispatch — WmsImageLayer and WmsElevationCoverage both advertise "WMS" but are disambiguated by the data_type discriminator.

Third-party layer types can register their own openers at startup; the registry is append-only and not thread-safe (intended use is one-shot init).