Cas Generic Getters

cas — content-addressing for non-git sources

CAS now covers module sources beyond git: http(s), Amazon S3, Google Cloud Storage, and Mercurial. Repeat runs against an unchanged remote reuse the cached tree instead of downloading the bytes again.

Before fetching, CAS issues a cheap remote probe (an HTTP HEAD, an S3 object-attributes lookup, a GCS metadata read, or hg identify) to derive a cache key without pulling the source. On a hit, the cached tree is linked directly; on a miss, or when the remote exposes no usable signal, CAS downloads the source, ingests it, and keys the resulting tree by its content hash. A remote that publishes a new version under the same address pins to a new entry, so a stale cache cannot serve outdated bytes.