Decide whether an actor may author the exact proposal union carried by a
commit. Admins may author any otherwise-valid union. A non-admin may author
only an empty/self-update commit, Updates sent exclusively by its own leaf,
or a SelfRemove-only union.
This is deliberately independent of MLS state: callers must derive the
actor identity and leaf index from the local state and pass the exact
by-reference plus by-value union that createCommit will encode.
Decide whether an actor may author the exact proposal union carried by a commit. Admins may author any otherwise-valid union. A non-admin may author only an empty/self-update commit, Updates sent exclusively by its own leaf, or a SelfRemove-only union.
This is deliberately independent of MLS state: callers must derive the actor identity and leaf index from the local state and pass the exact by-reference plus by-value union that
createCommitwill encode.