Cas Unit Source Preserves Subdir

update_source_with_cas: preserve //subdir on a unit's terraform.source

When a unit's terraform.source used the // subdir convention (for example, source = "../..//modules/foo") and opted into update_source_with_cas, the rewritten source dropped the //subdir tail and the synthetic tree contained only the leaf module's files. A module that referenced a sibling via a relative path (source = "../bar") could not resolve that reference after materialization.

Rewrites now preserve the original //subdir (for example, cas::sha1:<hash>//modules/foo), and the synthetic tree is rooted at the path before //, so sibling files reachable via relative paths land in the materialized working directory.

Sources without // are unchanged: the tree stays scoped to the leaf module, and the rewritten reference has no //subdir tail.