Creates an immutable commit (snapshot) of the Multi FileFeed's transforms.
A request body with the transforms payload is required.

The endpoint atomically:

1. Checks that the HEAD is not dirty (rejects with 422 if it is)
2. Replaces all nodes on the HEAD with the supplied payload
3. Creates an immutable commit snapshot

The dirty check prevents accidentally overwriting in-progress UI edits.
If the HEAD has uncommitted changes, the API consumer must coordinate
with the UI user before proceeding.

This is the primary endpoint for programmatic transform management.
Imports run against the latest commit. Immediately after this call,
the new commit becomes the latest commit unless another commit is
created before POST /imports.

Requires Multi FileFeed transforms to be enabled for the organization.