Add sandbox_c cmake project + ccache workflow for pool testing

sandbox_c/include/temperature.h

@@ -0,0 +1,12 @@
+#ifndef TEMPSENSE_H
+#define TEMPSENSE_H
+#include <stddef.h>
+
+typedef struct { char sensor[32]; double celsius; } Reading;
+
+double to_fahrenheit(double celsius);
+int parse_reading(const char *json, Reading *out);      /* 0 on success */
+int format_reading(const Reading *in, char *buf, size_t len);
+double clamp_to_absolute_zero(double celsius);
+
+#endif