perf(wled): cache per-frame max-pixel for brightness threshold

Cache the np.max(frame) result keyed on frame identity. The
min_brightness_threshold check ran a full reduction over the LED
array on every loop iteration even when the frame reference was
unchanged — at 60 fps with multiple targets this added up to
hundreds of redundant reductions per second.

server/src/ledgrab/core/processing/wled_target_processor.py

@@ -845,6 +845,11 @@ class WledTargetProcessor(TargetProcessor):
         prev_frame_ref = None
         has_any_frame = False
         _prev_brightness = -1  # force first send
+        # Cache max-pixel per frame ref. ``np.max`` over a 200-300 LED array
+        # at 60 fps × N targets is non-trivial; we only need to recompute it
+        # when the frame reference itself changes.
+        _max_pixel_cached: int = 0
+        _max_pixel_for_frame: object = None
 
         # Pre-allocate brightness scratch (uint16 intermediate + uint8 output)
         _bright_u16: Optional[np.ndarray] = None
@@ -1012,7 +1017,12 @@ class WledTargetProcessor(TargetProcessor):
                         # If below cutoff → snap to 0 (LEDs off).
                         _thresh = self._min_brightness_threshold
                         if _thresh > 0 and cur_brightness > 0:
-                            max_pixel = int(np.max(frame))
+                            if frame is _max_pixel_for_frame:
+                                max_pixel = _max_pixel_cached
+                            else:
+                                max_pixel = int(np.max(frame))
+                                _max_pixel_cached = max_pixel
+                                _max_pixel_for_frame = frame
                             if max_pixel * cur_brightness // 255 < _thresh:
                                 cur_brightness = 0