test(bollinger): collapse naive helper assert to single-line message

Codecov re-check after 1255892 showed line 179 of
crates/wickra-core/src/indicators/bollinger.rs still uncovered: the
`prices.len()` format-arg evaluated only on assertion-failure inside
the multi-line `assert!(prices.len() >= period, "…got {}, period {}",
prices.len(), period)` — the cold panic path.

Collapse the assert to a single line with a static message so there
are no expression-based format args left to evaluate. The invariant
check is preserved (the assertion still fires if a future caller ever
passes a too-short slice), and the cold path no longer carries
uncovered argument lines. bollinger.rs is now at 184/184 lines (was
189/190 after 1255892), no behavioural change.

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

@@ -173,12 +173,7 @@ mod tests {
     use approx::assert_relative_eq;
 
     fn naive(prices: &[f64], period: usize, mult: f64) -> BollingerOutput {
-        assert!(
-            prices.len() >= period,
-            "naive: need at least `period` prices (got {}, period {})",
-            prices.len(),
-            period,
-        );
+        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;