Cas Shared Unit Template

cas — update_source_with_cas now idempotent across unit and stack blocks

A terragrunt.stack.hcl with two blocks pointing at the same template directory used to fail stack generate when each block had update_source_with_cas = true. The first block's pass rewrote the shared template's source to a cas::sha256:... reference, then the second block's pass re-read the rewritten file and treated the reference as a relative path.

CAS now skips re-processing a source once it already carries the cas:: prefix, so multiple unit or stack blocks can share a template and resolve to the same synthetic tree.