feat(examples): extend hello with a second translation unit and a test

Gives the archive more than one member and gives meowc test something to run.

examples/hello/src/mathx/stats.c

@@ -0,0 +1,17 @@
+#include "mathx.h"
+#include <math.h>
+
+double stats_mean(const double *a, size_t n) {
+    if (n == 0) return 0.0;
+    double sum = 0.0;
+    for (size_t i = 0; i < n; i++) sum += a[i];
+    return sum / (double)n;
+}
+
+double stats_stddev(const double *a, size_t n) {
+    if (n < 2) return 0.0;
+    double m = stats_mean(a, n);
+    double acc = 0.0;
+    for (size_t i = 0; i < n; i++) acc += (a[i] - m) * (a[i] - m);
+    return sqrt(acc / (double)(n - 1));
+}