fix(io): CSV export writes the rows the view prunes (improvise-iot6)

`render_csv` built its layout with `GridLayout::new`, which does not apply
`prune_empty` — only `with_frozen_records` does. So `:export` in the TUI and
the CLI export path both wrote rows the grid does not draw. User-visible, in
production, and shipping for as long as the two constructors have differed.

The fourth instance of this trap; the third was `project::grid_in` in PR #78,
and this is that fix line for line.

The fixture is the difficulty, and the ticket says so. The sibling property
written for improvise-t6db passed on the buggy tree in its first version,
because it made its two views differ by turning pruning *off* — vacuous, since
with nothing to prune both code paths agree. So this one contains a row that is
actually empty, leaves `prune_empty` ON, and asserts the precondition in both
directions before exporting: `with_frozen_records` draws 2 rows where `new`
holds 3. If pruning ever becomes a no-op the precondition fails and says so,
rather than the assertion passing vacuously.

No fault needed injecting — the bug was live. On the unfixed tree the test
fails with `["Food", "Gas", "Rent"]` against `["Food", "Gas"]`. None of
`export.rs`'s three pre-existing tests mentions pruning at all, which is
exactly why this shipped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

context/test-evidence/formula-and-io.md

@@ -731,3 +731,34 @@ Case counts went 500 → 256 in the same file, which is the default
 a further 0.39s → 0.21s but it is not the fix, and reporting it as one would
 be the mistake this entry exists to record. Net: 1.42s → 0.21s, and
 `improvise-io` is no longer the run's longest binary.
+
+### export_csv_omits_the_rows_the_view_prunes
+
+improvise-iot6. `render_csv` built its layout with `GridLayout::new`, which does
+**not** apply `prune_empty` — only `GridLayout::with_frozen_records` does. So
+`:export` in the TUI and the CLI export path both wrote rows the grid does not
+draw. User-visible, in production, and shipping for as long as the two
+constructors have differed.
+
+This is the **fourth** instance of the same trap; the third was
+`project::grid_in`, fixed in PR #78, and this fix is that one line for line.
+No fault needed injecting: on the unfixed tree the test fails with
+
+> CSV wrote a row the grid does not draw: left `["Food", "Gas", "Rent"]`,
+> right `["Food", "Gas"]`
+
+**Why the fixture is discriminating, which is the whole difficulty here.** The
+sibling property written for improvise-t6db passed on the buggy tree in its
+first version, because it made its two views differ by turning pruning *off* —
+plenty of differing config, and vacuous, since with nothing to prune both code
+paths agree. So this fixture does two things that one cannot fake:
+
+- it contains a row that is **actually empty** (`Rent`, an item of `Type` that
+  no cell names) and leaves `prune_empty` **ON**;
+- it asserts the precondition in both directions before exporting anything —
+  `with_frozen_records` draws 2 rows and `new` holds 3. If a future change makes
+  pruning a no-op, or makes the two constructors agree, the precondition fails
+  and says so rather than the assertion passing vacuously.
+
+None of `export.rs`'s three pre-existing tests mentions pruning at all, which is
+exactly why this shipped.

crates/improvise-io/src/persistence/export.rs

@@ -30,7 +30,9 @@ fn render_csv(workbook: &Workbook, view_name: &str) -> Result<Vec<u8>> {
         .get(view_name)
         .ok_or_else(|| anyhow::anyhow!("View '{view_name}' not found"))?;
 
-    let layout = GridLayout::new(&workbook.model, view);
+    // `with_frozen_records`, not `new`: `new` skips `prune_empty`, so an export
+    // writes rows the grid does not draw (improvise-iot6).
+    let layout = GridLayout::with_frozen_records(&workbook.model, view, None);
     let model = &workbook.model;
     let (fmt_comma, fmt_decimals) = crate::format::parse_number_format(&view.number_format);
 
@@ -144,6 +146,63 @@ mod tests {
         );
     }
 
+    /// Bug improvise-iot6: `render_csv` built its layout with `GridLayout::new`,
+    /// which never applies `prune_empty`. Evidence: `context/test-evidence/formula-and-io.md`.
+    #[test]
+    fn export_csv_omits_the_rows_the_view_prunes() {
+        use crate::view::{Axis, GridLayout};
+        let mut wb = Workbook::new("T");
+        wb.add_category("Type").unwrap();
+        wb.add_category("Month").unwrap();
+        for item in ["Food", "Gas", "Rent"] {
+            wb.add_item("Type", item).unwrap();
+        }
+        wb.add_item("Month", "Jan").unwrap();
+        // `Rent` is the empty row: every other Type carries a Jan value.
+        for t in ["Food", "Gas"] {
+            wb.model.set_cell(
+                coord(&[("Type", t), ("Month", "Jan")]),
+                CellValue::Number(10.0),
+            );
+        }
+        {
+            let v = wb.active_view_mut();
+            v.set_axis("Type", Axis::Row);
+            v.set_axis("Month", Axis::Column);
+            v.prune_empty = true;
+        }
+
+        // The precondition, or this test proves nothing: pruning must really be
+        // ON and must really drop a row the model still holds an item for.
+        assert!(wb.active_view().prune_empty, "pruning must stay ON");
+        let drawn = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
+        assert_eq!(
+            drawn.row_count(),
+            2,
+            "the view must draw two of the three rows"
+        );
+        assert_eq!(
+            GridLayout::new(&wb.model, wb.active_view()).row_count(),
+            3,
+            "the unpruned layout must really hold the third row"
+        );
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("export.csv");
+        super::export_csv(&wb, "Default", &path).unwrap();
+        let mut rdr = csv::ReaderBuilder::new()
+            .has_headers(false)
+            .from_path(&path)
+            .unwrap();
+        let records: Vec<csv::StringRecord> = rdr.records().collect::<Result<_, _>>().unwrap();
+        let labels: Vec<&str> = records.iter().skip(1).map(|r| &r[0]).collect();
+        assert_eq!(
+            labels,
+            ["Food", "Gas"],
+            "CSV wrote a row the grid does not draw"
+        );
+    }
+
     #[test]
     fn export_csv_unknown_view_returns_error() {
         let m = two_cat_model();