fix(data): expose actual snapshot row counts without cloning data

crates/fidc-core/src/data.rs

@@ -2250,6 +2250,17 @@ impl DataSet {
             .unwrap_or(&[])
     }
 
+    /// Counts market, factor, candidate, benchmark and corporate-action rows without cloning them.
+    pub fn snapshot_row_counts(&self) -> (usize, usize, usize, usize, usize) {
+        (
+            self.market_by_date.values().map(Vec::len).sum(),
+            self.factor_by_date.values().map(Vec::len).sum(),
+            self.candidate_by_date.values().map(Vec::len).sum(),
+            self.benchmark_by_date.len(),
+            self.corporate_actions_by_date.values().map(Vec::len).sum(),
+        )
+    }
+
     pub fn execution_quotes_on(&self, date: NaiveDate, symbol: &str) -> &[IntradayExecutionQuote] {
         self.execution_quotes_by_date
             .get(&date)
@@ -5474,6 +5485,10 @@ mod tests {
         )
         .expect("daily bundle dataset");
 
+        let row_count = dates.len() * symbols.len();
+        let expected_counts = (row_count, row_count, row_count, dates.len(), dates.len());
+        assert_eq!(flat.snapshot_row_counts(), expected_counts);
+        assert_eq!(grouped.snapshot_row_counts(), expected_counts);
         assert_eq!(flat.calendar().days(), grouped.calendar().days());
         assert_eq!(flat.benchmark_code(), grouped.benchmark_code());
         for date in dates {