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/command/dispatch.rs

@@ -59,7 +59,7 @@ pub fn dispatch(model: &mut Model, cmd: &Command) -> CommandResult {
                 .map(|pair| (pair[0].clone(), pair[1].clone()))
                 .collect();
             let key = CellKey::new(kv);
-            model.set_cell(key, CellValue::Empty);
+            model.clear_cell(&key);
             CommandResult::ok()
         }