Polymorphism Phase 2 + remove unused gamma/saturation fields

ProcessorManager: replace all isinstance checks with property-based
dispatch via base TargetProcessor (device_id, led_client,
get_display_index, update_device, update_calibration).

Remove gamma/saturation from ProcessingSettings, ColorCorrection
schema, serialization, and migration — these were never used in the
processing pipeline and are handled by postprocessing template filters.
Delete dead apply_color_correction() function.

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

docs/API.md

@@ -201,12 +201,11 @@ Get processing settings.
 {
   "display_index": 0,
   "fps": 30,
-  "border_width": 10,
-  "color_correction": {
-    "gamma": 2.2,
-    "saturation": 1.0,
-    "brightness": 1.0
-  }
+  "brightness": 1.0,
+  "smoothing": 0.3,
+  "interpolation_mode": "average",
+  "standby_interval": 1.0,
+  "state_check_interval": 30
 }
 ```
 
@@ -219,12 +218,7 @@ Update processing settings.
 {
   "display_index": 1,
   "fps": 60,
-  "border_width": 15,
-  "color_correction": {
-    "gamma": 2.4,
-    "saturation": 1.2,
-    "brightness": 0.8
-  }
+  "brightness": 0.8
 }
 ```