The Iteration-2 rule wrote a header-only CSV for every existing
table, which surprised users who created a table and saw a file
appear before any data went in. Tighten the rule: a CSV exists
iff the table has rows. Persistence::write_table_data now
delegates to delete_table_data when the snapshot is empty,
removing any prior CSV. The schema-only invariant (YAML knows the
table; CSV knows its rows) is preserved.
The cascade-delete integration test was rewritten to assert the
CSVs vanish; two new tests pin the rule (create -> no CSV;
delete --all-rows -> CSV removed).
Tests: 291 passing (256 lib + 9 + 9 + 17), 0 failing, 0 skipped.
claude@clouddev1
5410075398