Add brightness temporal smoothing to reduce oscillation from moving objects

- Added brightness-smoothing parameter (0-1, default 0.1)
- Implements exponential moving average to filter transient brightness changes
- Samples brightness every frame but smooths before adjusting exposure
- Reduces oscillation from people/cars/birds moving through scene
- Updated DEBUG.md with complete implementation details

Recommended settings for dawn/dusk time-lapse:
  ramp-rate=vslow update-interval=1000 brightness-smoothing=0.1

gst/intervalometer/gstintervalometer.h

@@ -74,6 +74,7 @@ struct _GstIntervalometer
   gchar *log_file;               /* CSV log file path */
   gchar *camera_element_name;    /* Name of upstream idsueyesrc element */
   guint update_interval;         /* Update interval in milliseconds */
+  gdouble brightness_smoothing;  /* Brightness smoothing factor (0-1, 0=no smoothing) */
 
   /* Internal state */
   GstElement *camera_src;        /* Reference to upstream camera element */
@@ -94,6 +95,10 @@ struct _GstIntervalometer
   GstVideoInfo video_info;
   gboolean video_info_valid;
   
+  /* Brightness smoothing */
+  gdouble smoothed_brightness;   /* Exponentially smoothed brightness value */
+  gboolean brightness_initialized; /* Whether smoothed_brightness has been initialized */
+  
   /* Ramping parameters */
   gdouble ramp_step;             /* Current ramping step size */
 };