refactor: eliminate Box<dyn Iterator> and Option sentinels in export_csv

The CSV export used Box<dyn Iterator<Item = Option<usize>>> to unify
empty and non-empty row iteration, violating the CLAUDE.md rule that
Box/Rc container management should be split from logic. Replaced with a
direct for loop over row indices, removing both the Box and the Option
sentinels used to represent "placeholder empty row/col".

Also removes unused pub use cell::CellKey re-export and an unused
import in cell.rs tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/model/cell.rs

@@ -143,7 +143,7 @@ impl DataStore {
 
 #[cfg(test)]
 mod cell_key {
-    use super::{CellKey, CellValue, DataStore};
+    use super::CellKey;
 
     fn key(pairs: &[(&str, &str)]) -> CellKey {
         CellKey::new(pairs.iter().map(|(c, i)| (c.to_string(), i.to_string())).collect())

src/model/mod.rs

@@ -2,5 +2,4 @@ pub mod category;
 pub mod cell;
 pub mod model;
 
-pub use cell::CellKey;
 pub use model::Model;

src/persistence/mod.rs

@@ -79,33 +79,19 @@ pub fn export_csv(model: &Model, view_name: &str, path: &Path) -> Result<()> {
     out.push_str(&col_labels.join(","));
     out.push('\n');
 
-    // Data rows — treat zero-item axes as a single empty placeholder row/col
-    let row_range: Box<dyn Iterator<Item = Option<usize>>> = if layout.row_count() == 0 {
-        Box::new(std::iter::once(None))
-    } else {
-        Box::new((0..layout.row_count()).map(Some))
-    };
-    let col_indices: Vec<Option<usize>> = if layout.col_count() == 0 {
-        vec![None]
-    } else {
-        (0..layout.col_count()).map(Some).collect()
-    };
-
-    for row_opt in row_range {
-        let row_label = row_opt.map(|ri| layout.row_label(ri)).unwrap_or_default();
+    // Data rows
+    for ri in 0..layout.row_count() {
+        let row_label = layout.row_label(ri);
         if !row_label.is_empty() {
             out.push_str(&row_label);
             out.push(',');
         }
-        let row_values: Vec<String> = col_indices.iter().map(|&col_opt| {
-            match (row_opt, col_opt) {
-                (Some(ri), Some(ci)) => layout.cell_key(ri, ci)
+        let row_values: Vec<String> = (0..layout.col_count())
+            .map(|ci| layout.cell_key(ri, ci)
                 .and_then(|key| model.evaluate(&key))
                 .map(|v| v.to_string())
-                    .unwrap_or_default(),
-                _ => String::new(),
-            }
-        }).collect();
+                .unwrap_or_default())
+            .collect();
         out.push_str(&row_values.join(","));
         out.push('\n');
     }