test(incremental): hidden members under every aggregation, evidence and the step 4 reasoning (improvise-u2le)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

crates/improvise-incremental/tests/measure.rs

@@ -181,3 +181,18 @@ fn occupancy_with_a_hidden_item_reads_the_groups_it_subtracts() {
         "the line, less p3 under it"
     );
 }
+
+#[test]
+fn a_group_whose_cells_are_all_cleared_is_no_longer_read() {
+    let mut backend = Incremental::new(table(2, AggFunc::Count), vec![]);
+    let one = cell(1, 1, 1);
+    assert_eq!(backend.raw_fold(&one), Some(CellValue::Number(1.0)));
+    backend.apply(&Edit::ClearCell(cell(1, 1, 1)));
+    let before = backend.meter();
+    assert_eq!(backend.raw_fold(&one), None, "nothing is left under it");
+    assert_eq!(
+        since(before, backend.meter()).reads,
+        0,
+        "and no empty group stands in for it"
+    );
+}