Cas Literal Source Validation

cas — update_source_with_cas requires a literal source string

When a catalog unit, stack, or terraform block set update_source_with_cas = true with a source that was not a literal string, rewriting silently produced a wrong source. Interpolation such as "../units/${local.name}" had the interpolated portion dropped, leaving a bare prefix; a reference such as local.foo resolved to the directory containing the block itself. In both cases stack generation packaged the wrong directory without any error.

Stack generation now fails with an error explaining that update_source_with_cas requires a literal source string. Non-literal expressions, including interpolation, function calls, and references like local.foo, are rejected.