Refactor core/ into logical sub-packages and split filter files

Reorganize the flat core/ directory (17 files) into three sub-packages:
- core/devices/ — LED device communication (led_client, wled/adalight clients, providers, DDP)
- core/processing/ — target processing pipeline (processor_manager, target processors, live streams, settings)
- core/capture/ — screen capture & calibration (screen_capture, calibration, pixel_processor, overlay)

Also split the monolithic filters/builtin.py (460 lines, 8 filters) into
individual files: brightness, saturation, gamma, downscaler, pixelate,
auto_crop, flip, color_correction.

Includes the ProcessorManager refactor from target-centric architecture:
ProcessorManager slimmed from ~1600 to ~490 lines with unified
_processors dict replacing duplicate _targets/_kc_targets dicts.

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

server/tests/test_calibration.py

@@ -3,7 +3,7 @@
 import numpy as np
 import pytest
 
-from wled_controller.core.calibration import (
+from wled_controller.core.capture.calibration import (
     CalibrationSegment,
     CalibrationConfig,
     PixelMapper,
@@ -13,7 +13,7 @@ from wled_controller.core.calibration import (
     EDGE_ORDER,
     EDGE_REVERSE,
 )
-from wled_controller.core.screen_capture import BorderPixels
+from wled_controller.core.capture.screen_capture import BorderPixels
 
 
 def test_calibration_segment():