feat: add fps module for rolling frame/fps diagnostics

Real-time 1-second rolling window, independent of tick rate or
display refresh.

include/fps.h

@@ -0,0 +1,21 @@
+#ifndef SKELE_FPS_H
+#define SKELE_FPS_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern float skele_fps;
+extern float skele_frame_time_ms;
+extern float skele_frame_budget_ms;
+
+void skele_fps_init(void);
+void skele_fps_frame_mark(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SKELE_FPS_H */