Xcode CAS artifact cap raised from 25 MiB to 256 MiB

Kura now accepts Xcode compilation cache artifacts up to 256 MiB, up from the previous 25 MiB cap. App-shaped projects hit that ceiling with fat debug-info objects, asset-catalog outputs, and bundles, which caused uploads to fail and left those tasks in a permanent-miss set that recompiled and retried on every build. Raising the cap lets those large artifacts publish, so Xcode cache hit rates can reach ~100% and warm cached builds stop being slower than building with no cache.