On a tz-change full re-derive, mergeDayEntries carried a baseline slice
whenever the fresh day had no data slice for that (date, provider), so a
turn that re-bucketed across local midnight left its old day sliceless,
got carried there, AND counted again on its new day. This subtracts from
each carried baseline slice exactly what the fresh parse still attributes
to that (date, provider) under the OLD bucketing (dateKeyInTz): the
re-bucketed turns, nothing else. A sources-gone slice has no such content
and survives untouched; a fully-explained slice is dropped; residual
slices ADD their sessions instead of max-dedup, since the subtraction
already removed the placeholder's share.
