isCacheOnly

When true, the factory never calls networkSource — cache hits are served as usual; cache misses return null from fetchElevationData. Toggleable at runtime so a single layer can flip between online and offline modes without re-wiring its source factory. Shares the earth.worldwind.layer.cache.OfflineToggleable interface with earth.worldwind.layer.cache.CachedTileSource so app-level offline toggles dispatch generically.