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/tests/mathx_test.c

@@ -0,0 +1,28 @@
+#include "mathx.h"
+#include <math.h>
+#include <stdio.h>
+
+static int failures = 0;
+
+static void check(const char *what, double got, double want) {
+    if (fabs(got - want) > 1e-9) {
+        printf("  %s: got %.12f, want %.12f\n", what, got, want);
+        failures++;
+    }
+}
+
+int main(void) {
+    const double a[] = {1.0, 2.0, 3.0, 4.0};
+    const double b[] = {4.0, 3.0, 2.0, 1.0};
+    const double unit[] = {3.0, 4.0};
+
+    check("vec_dot", vec_dot(a, b, 4), 20.0);
+    check("vec_sum", vec_sum(a, 4), 10.0);
+    check("vec_norm", vec_norm(unit, 2), 5.0);
+    check("stats_mean", stats_mean(a, 4), 2.5);
+    check("stats_stddev", stats_stddev(a, 4), sqrt(5.0 / 3.0));
+    check("stats_mean empty", stats_mean(a, 0), 0.0);
+    check("stats_stddev single", stats_stddev(a, 1), 0.0);
+
+    return failures == 0 ? 0 : 1;
+}