style(bollinger): wrap naive helper assert to satisfy rustfmt

Commit aa2846c collapsed the multi-line assert in the test-only naive
helper to a single line to drop the cold expression-arg lines that
Codecov saw as uncovered. The single-line form exceeded the 100-col
limit, so cargo fmt --check failed on every supported toolchain in CI
(ubuntu-latest, macos-latest, windows-latest).

Let rustfmt wrap it back to the three-line form. The arms are still
just a literal expression and a static-string message — no expression
format args — so the cold-path lines that Codecov originally flagged
on line 179 stay covered. No behaviour change.

crates/wickra-core/src/indicators/bollinger.rs

@@ -173,7 +173,10 @@ mod tests {
     use approx::assert_relative_eq;
 
     fn naive(prices: &[f64], period: usize, mult: f64) -> BollingerOutput {
-        assert!(prices.len() >= period, "naive requires at least `period` prices");
+        assert!(
+            prices.len() >= period,
+            "naive requires at least `period` prices"
+        );
         let w = &prices[prices.len() - period..];
         let mean = w.iter().sum::<f64>() / period as f64;
         let var = w.iter().map(|x| (x - mean).powi(2)).sum::<f64>() / period as f64;