Applies an eventstream delta to a cached resource.

Maps recurse; everything else replaces.

The distinction is load-bearing rather than stylistic. CLIP v2 events are partial — an observed light event carried on: %{"on" => true} and the two identity fields, and nothing else — so replacing the cached resource with the delta would erase the light's brightness, colour, and capabilities. Merging one level deep is not enough either: a delta carrying %{"dimming" => %{"brightness" => 86.11}} must leave the min_dim_level cached beside it alone.

Lists replace because a list arriving in an event is authoritative. A services list is the complete set of that device's services, not an addition to it, and there is no key to merge list elements on.

A struct replaces because it is treated as an opaque value, the same as a list or a scalar. Deep-merging two structs key by key produces a map carrying __struct__ but not that struct's actual field set — something that is neither struct. Nothing arriving from the bridge is ever a struct; a struct reaching here means a caller passed something this function was not built to merge, and replacing it whole is the only treatment that does not silently fabricate a hybrid.