style: use arrays rather than vec! in the calibration fixture

clippy::useless_vec. Pushed broken because the verification chain used
`if ... then echo` blocks that report status without gating the `&&`
that follows — the same shape of mistake as e1bddf2, where a grep
succeeded on the error text. Both times the check printed FAIL and the
push went ahead.

The reliable form is a single fail-fast chain:

    just lint && just test && just determinism && cargo test --doc

so nothing runs after the first failure.

tests/marginal_calibration.rs

@@ -100,7 +100,7 @@ fn inverse(mut a: Vec<Vec<f64>>) -> Vec<Vec<f64>> {
 /// precision = prior precision + sum of a_k a_k^T / v_k.
 fn exact_for(obs: &[(usize, usize, f64)]) -> (Vec<f64>, Vec<Vec<f64>>) {
     let mut lambda = vec![vec![0.0; N]; N];
-    let mut eta = vec![0.0; N];
+    let mut eta = [0.0; N];
     for (i, row) in lambda.iter_mut().enumerate() {
         row[i] = 1.0 / (SIGMA0 * SIGMA0);
         eta[i] = MU0 / (SIGMA0 * SIGMA0);
@@ -109,7 +109,7 @@ fn exact_for(obs: &[(usize, usize, f64)]) -> (Vec<f64>, Vec<Vec<f64>>) {
     // Each 1v1 observation: d ~ N(x_a - x_b, score_sigma^2 + 2 beta^2)
     let v = SCORE_SIGMA * SCORE_SIGMA + 2.0 * BETA * BETA;
     for &(a, b, d) in obs {
-        let mut vec_a = vec![0.0; N];
+        let mut vec_a = [0.0; N];
         vec_a[a] = 1.0;
         vec_a[b] = -1.0;
         for i in 0..N {