use a constant for sleep delay

ambientd.c3

@@ -18,6 +18,8 @@ const int MAX_LUMINANCE = 700;
 // minimum display brightness in percent
 const float MIN_BRIGHTNESS_PERCENT = 0.05;
 
+const int SLEEP_DELAY = 3000;
+
 macro abs(x) { return x < 0 ? -x : x; }
 macro clamp(x, l, h) { return max(min(x, h), l); }
 
@@ -155,7 +157,7 @@ fn int main()
 		  io::eprintfn("Error writing brightness: %s", excuse);
 		}
 
-		thread::sleep_ms(3000);
+		thread::sleep_ms(SLEEP_DELAY);
 
 		// read actual brightness to get the user offset
 		int! actual_brightness;