Shipped content is kept on disk at its exact commit

Content that ships from a git repository already arrives with a commit identity, but nothing kept that identity on disk — so anything derived from that content had to be recomputed without a cheap way to ask "has this actually changed?".

A portal can now keep one checkout per repository and, beside it, a narrow checkout per module pinned to the exact commit it was synced from. Asking for a module at a commit already on disk does no work at all, and each module's checkout contains only that module rather than a copy of the whole repository. Superseded commits are removed when the caller that created them says so, rather than accumulating.

Nothing depends on it yet — this is the foundation for making compilation skip modules whose sources have not moved. The commit is the version key that makes "unchanged" answerable without inspecting every file.