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;