The auto-generated semantic-layer overlay description embedded
measure/segment/column counts that were computed once and never
recomputed, so the summary drifted and misreported its source after
measures were later appended. Make the auto fallback count-free, since
those counts are already rendered live from the body at `ktx sl list`/
`read` time; this removes the drift class without ever overwriting
human-authored descriptions (the fill-only-when-empty guard is untouched).
Adds a regression test that fails on main and passes after the fix, plus
guards for description preservation and the no-measures fallback.
