A maintenance snapshot carried only its own labels — merged-files,
delete-groups and friends — and no summary counters, so every engine that
reads a table's size out of the current snapshot summary reported nothing
for it: PyIceberg's inspect.snapshots, Trino's $snapshots and Spark's
DESCRIBE all read total-records, total-data-files and total-files-size
verbatim, and a table lost them the moment compaction touched it.
Accumulate the files each operation adds and removes, and render them the
way the spec defines: the added-*/removed-* counters from the files
themselves, then the totals carried over from the parent snapshot.
Carry a total only when the parent recorded it. Iceberg treats a missing
total as zero, which turns a compaction replacing two files with one into a
negative total-data-files, or a table with millions of rows into
total-records: 0. Leaving the field out lets a reader fall back to the
manifests instead of believing a made-up number.
Compaction also accounts for the delete files it consumes, so a run that
folds every delete into the rewritten data reports them as removed.
