refactor: replace CellValue::Empty with Option<CellValue>

Previously CellValue had three variants: Number, Text, and Empty.
The Empty variant acted as a null sentinel, but the compiler could not
distinguish between "this is a real value" and "this might be empty".
Code that received a CellValue could use it without checking for Empty,
because there was no type-level enforcement.

Now CellValue has only Number and Text. The absence of a value is
represented as None at every API boundary:

  DataStore::get()    → Option<&CellValue>  (was &CellValue / Empty)
  Model::get_cell()   → Option<&CellValue>  (was &CellValue / Empty)
  Model::evaluate()   → Option<CellValue>   (was CellValue::Empty)
  eval_formula()      → Option<CellValue>   (was CellValue::Empty)

Model gains clear_cell() for explicit key removal; ClearCell dispatch
calls it instead of set_cell(key, CellValue::Empty).

The compiler now forces every caller of evaluate/get_cell to handle
the None case explicitly — accidental use of an empty value as if it
were real is caught at compile time rather than silently computing
wrong results.

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

src/persistence/mod.rs

@@ -100,7 +100,8 @@ pub fn export_csv(model: &Model, view_name: &str, path: &Path) -> Result<()> {
         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)
-                    .map(|key| model.evaluate(&key).to_string())
+                    .and_then(|key| model.evaluate(&key))
+                    .map(|v| v.to_string())
                     .unwrap_or_default(),
                 _ => String::new(),
             }