commitAssignment and moveEntry stored the same spec object in both
stack.spec and entry.spec. A later, more complete scan enriching
stack.spec would retroactively mutate the entry that first created the
stack, making it appear as if it had been scanned with data it never
actually had. Both places now store independent shallow copies so the
entries log and the stacks spec (comparison baseline) can't leak into
each other.
Also documents that undoLast intentionally reverts the last *recorded*
entry, not the last *action* - a subsequent moveEntry/removeEntry on a
different entry does not change what undoLast will take back - and
adds a test pinning that contract after a reorder.
vchuser