Fix serial send bloat when sharing CSS stream with higher-LED device

When two targets share the same color strip source, configure() resizes
the stream to the last caller's LED count. If a WLED device (934 LEDs)
starts after a serial device (fewer LEDs), the serial target sends the
full 934-LED frame over serial, massively inflating send time.

Add _fit_to_device() to truncate/tile colors to the target's actual
device LED count before sending, so each consumer only transmits what
its device needs regardless of the shared stream's current size.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -362,6 +362,22 @@ class WledTargetProcessor(TargetProcessor):
             return (colors.astype(np.uint16) * device_info.software_brightness >> 8).astype(np.uint8)
         return colors
 
+    @staticmethod
+    def _fit_to_device(colors: np.ndarray, device_led_count: int) -> np.ndarray:
+        """Truncate or repeat colors to match the target device LED count.
+
+        Shared streams may be sized to a different consumer's LED count.
+        This ensures each target only sends what its device actually needs.
+        """
+        n = len(colors)
+        if n == device_led_count or device_led_count <= 0:
+            return colors
+        if n > device_led_count:
+            return colors[:device_led_count]
+        # Tile to fill (rare — usually streams are >= device count)
+        reps = (device_led_count + n - 1) // n
+        return np.tile(colors, (reps, 1))[:device_led_count]
+
     async def _processing_loop(self) -> None:
         """Main processing loop — poll ColorStripStream → apply brightness → send."""
         stream = self._color_strip_stream
@@ -430,7 +446,10 @@ class WledTargetProcessor(TargetProcessor):
                             if prev_colors is not None and (loop_start - last_send_time) >= standby_interval:
                                 if not self._is_running or self._led_client is None:
                                     break
-                                send_colors = self._apply_brightness(prev_colors, device_info)
+                                kc = prev_colors
+                                if device_info and device_info.led_count > 0:
+                                    kc = self._fit_to_device(kc, device_info.led_count)
+                                send_colors = self._apply_brightness(kc, device_info)
                                 if self._led_client.supports_fast_send:
                                     self._led_client.send_pixels_fast(send_colors)
                                 else:
@@ -448,6 +467,10 @@ class WledTargetProcessor(TargetProcessor):
 
                         prev_colors = colors
 
+                        # Fit to this device's LED count (stream may be shared)
+                        if device_info and device_info.led_count > 0:
+                            colors = self._fit_to_device(colors, device_info.led_count)
+
                         # Apply device software brightness
                         send_colors = self._apply_brightness(colors, device_info)