feat: add --show-events overlay with raw log intensity

Visualize raw temporal brightness change (threshold=0, log domain)
as green(+)/red(-) gradient overlay proportional to |change|.
Supports video output and live display modes.
Enables EventProcessor threshold=0 for raw mode without clipping.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/event_utils.py

@@ -71,9 +71,14 @@ class EventProcessor:
             frame: np.ndarray, shape (H, W) or (H, W, C), uint8 or float.
 
         Returns:
-            events_binary:   np.ndarray (H, W), values in {-1, 0, +1}
-            events_strength: np.ndarray (H, W), values in [-1, 1]
-            event_count:     int, number of non-zero events
+            When threshold > 0:
+                events_binary:   np.ndarray (H, W), values in {-1, 0, +1}
+                events_strength: np.ndarray (H, W), values in [-1, 1]
+                event_count:     int, number of non-zero events
+            When threshold == 0 (raw output, no thresholding):
+                change_raw:  np.ndarray (H, W), raw log/linear brightness change (float32)
+                change_raw:  same as above
+                event_count: int, number of pixels with non-zero change
         """
         brightness = self._to_grayscale(frame)
 
@@ -81,10 +86,18 @@ class EventProcessor:
         if self.prev_brightness is None:
             self.prev_brightness = brightness
             h, w = brightness.shape
+            if self.threshold == 0:
+                return np.zeros((h, w), dtype=np.float32), np.zeros((h, w), dtype=np.float32), 0
             return np.zeros((h, w), dtype=np.int8), np.zeros((h, w), dtype=np.float32), 0
 
         change = self._compute_change(brightness)
 
+        # threshold == 0: raw mode, skip thresholding
+        if self.threshold == 0:
+            self.prev_brightness = brightness
+            change_f32 = change.astype(np.float32)
+            return change_f32, change_f32, int(np.count_nonzero(change))
+
         if self.auto_threshold:
             self._update_auto_threshold(change)