Add sandbox_c cmake project + ccache workflow for pool testing

sandbox_c/src/format.c

@@ -0,0 +1,7 @@
+#include "temperature.h"
+#include <stdio.h>
+int format_reading(const Reading *in, char *buf, size_t len) {
+    if (!in || !buf || len < 16) return 1;
+    int n = snprintf(buf, len, "{\"sensor\":\"%s\",\"celsius\":%.2f}", in->sensor, in->celsius);
+    return (n < 0 || (size_t)n >= len) ? 2 : 0;
+}