Persistence: empty table -> no CSV (per Iteration 2 follow-up)

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.

src/persistence/mod.rs

@@ -155,7 +155,19 @@ impl Persistence {
     /// Write `data/<table>.csv` from a table snapshot. Atomic
     /// per file. Creates the `data/` directory if missing
     /// (tolerant of fresh projects).
+    ///
+    /// **Empty tables produce no CSV.** A header-only file
+    /// would carry no information beyond what `project.yaml`
+    /// already records, so an empty snapshot is treated
+    /// identically to "drop this table's data file": the CSV
+    /// is removed if it exists, no file is created if it
+    /// doesn't. This keeps the rule "data lives in CSV; no
+    /// data, no CSV" consistent and avoids surprising users
+    /// with files they didn't ask for.
     pub fn write_table_data(&self, table: &TableSnapshot) -> Result<(), PersistenceError> {
+        if table.rows.is_empty() {
+            return self.delete_table_data(&table.name);
+        }
         let data_dir = self.project_path.join(DATA_DIR);
         fs::create_dir_all(&data_dir).map_err(|source| PersistenceError::Io {
             operation: "create",