Kura CAS cache capacity now scales with disk size and can be configured

Kura’s content-addressable storage (CAS) segment ring now derives its artifact-body capacity from the data directory’s filesystem size instead of using a fixed ~2.5 GiB hard-coded limit. By default it uses 50% of the total filesystem bytes, capped at 80% to avoid running the disk full. Operators can also set an explicit byte budget via the new KURA_CAS_CAPACITY_BYTES environment variable or config.casCapacityBytes in the Helm chart. On typical managed deployments with 10 GiB volumes, the default cache capacity roughly doubles from 2.5 GiB to 5 GiB, reducing ring rotation and improving cache hit rates for large builds. The previous 2.5 GiB floor remains for very small disks or when filesystem size cannot be determined.