Restore the loop count that loops_among's refactor dropped

The S3776 split moved the loop-detection body into loops_among without
the 'let n = circ.len()' it reads, so calc failed to compile and the
Sonar build broke. Restore the binding from main.

crates/calc/src/recalc.rs

@@ -1025,6 +1025,7 @@ impl Calc {
         if circ.is_empty() {
             return Vec::new();
         }
+        let n = circ.len();
         let index: HashMap<Key, usize> = circ.iter().enumerate().map(|(i, k)| (*k, i)).collect();
         // Edges from a cell to the `#CIRC!` formulas that read it; loops are the same either way.
         let mut deps = Vec::new();