feat: add fps module for rolling frame/fps diagnostics

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

bmake.mk

@@ -3,7 +3,7 @@ INPUT_SRC = src/platform/client/input/sdl.c
 
 .include "config.mk"
 
-ENGINE_SRCS      = src/skele.c ${CLOCK_SRC}
+ENGINE_SRCS      = src/skele.c src/fps.c ${CLOCK_SRC}
 SOFT_PLAT_SRCS   = src/platform/client/video/sdl.c ${INPUT_SRC}
 GL_PLAT_SRCS     = src/platform/client/video/gl.c ${INPUT_SRC}
 SOFT_CLIENT_SRCS = example/client/main.c

gmake.mk

@@ -8,7 +8,7 @@ endif
 
 include config.mk
 
-ENGINE_SRCS      := src/skele.c $(CLOCK_SRC)
+ENGINE_SRCS      := src/skele.c src/fps.c $(CLOCK_SRC)
 SOFT_PLAT_SRCS   := src/platform/client/video/sdl.c $(INPUT_SRC)
 GL_PLAT_SRCS     := src/platform/client/video/gl.c $(INPUT_SRC)
 SOFT_CLIENT_SRCS := example/client/main.c

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 */

src/fps.c

@@ -0,0 +1,41 @@
+#include "fps.h"
+#include "clock.h"
+
+float skele_fps = 0.0f;
+float skele_frame_time_ms = 0.0f;
+float skele_frame_budget_ms = 0.0f;
+
+static uint64_t win_start = 0;
+static uint64_t last_frame_ns = 0;
+static uint64_t frame_ns_accum = 0;
+static uint32_t frame_count = 0;
+
+void skele_fps_init(void)
+{
+	win_start = skele_time_ns();
+	last_frame_ns = win_start;
+	frame_ns_accum = 0;
+	frame_count = 0;
+	skele_fps = 0.0f;
+	skele_frame_time_ms = 0.0f;
+}
+
+void skele_fps_frame_mark(void)
+{
+	uint64_t now = skele_time_ns();
+
+	frame_ns_accum += now - last_frame_ns;
+	last_frame_ns = now;
+	frame_count++;
+
+	if (now - win_start >= 1000000000ULL) {
+		double elapsed_s = (double)(now - win_start) / 1e9;
+		skele_fps = (float)(frame_count / elapsed_s);
+		skele_frame_time_ms =
+			(float)((frame_ns_accum / (double)frame_count) / 1e6);
+
+		win_start = now;
+		frame_ns_accum = 0;
+		frame_count = 0;
+	}
+}