Revert "Remove @retroactive for now"

This reverts commit f49b1f2.

Author: bradhowes

Packages/Sources/KernelBridge/KernelBridge.swift

@@ -5,4 +5,4 @@ import Kernel
 
 // Extend the Obj-C KernelBridge with protocols for interoperability with `FilterAudioUnit` and `AudioUnitParameters`
 // classes.
-extension KernelBridge: AUParameterHandler, AudioRenderer {}
+extension KernelBridge: @retroactive AUParameterHandler, @retroactive AudioRenderer {}

Packages/Sources/Parameters/Parameters.swift

@@ -117,7 +117,7 @@ extension Parameters {
   }
 }
 
-extension AUParameter: AUParameterFormatting {
+extension AUParameter: @retroactive AUParameterFormatting {
 
   /// Obtain string to use to separate a formatted value from its units name
   public var unitSeparator: String {

iOS/Extension/ViewController.swift

@@ -12,7 +12,7 @@ import Theme
 
 import os.log
 
-extension Knob: AUParameterValueProvider, RangedControl {}
+extension Knob: @retroactive AUParameterValueProvider, @retroactive RangedControl {}
 
 /**
  Controller for the AUv3 filter view. Handles wiring up of the controls with AUParameter settings.

macOS/Extension/ViewController.swift

@@ -11,7 +11,7 @@ import ParameterAddress
 import Parameters
 import os.log
 
-extension Knob: AUParameterValueProvider, RangedControl {}
+extension Knob: @retroactive AUParameterValueProvider, @retroactive RangedControl {}
 
 /**
  Controller for the AUv3 filter view. Handles wiring up of the controls with AUParameter settings.