CacheReadableElevationSourceFactory

Optional capability for cache-backed ElevationSourceFactorys: read one tile from the local cache only, never touching the network. Lets a coverage serve cached tiles without spending a slot in the (small) network-retrieval budget, so a slow network DEM fetch for one uncached tile can't stall cached neighbours behind it. Returns a fully built ElevationImage on a cache hit, or null on a cache miss / decode failure.

Implementations must do the decode AND the ElevationImage min/max/missing scan off the render thread (inside their existing cache-read dispatch), so the caller resumes on the main thread with a ready image and only pays a single coroutine switch per cached tile.

Inheritors