Extracted the PreviewPreinstalling service in the Sample app target into its own file and declared it as development asset.

Apps/Attendi/Tests/Mocks/PreinstallingMock.swift

@@ -5,6 +5,8 @@ import Recording
 /// on demand.
 @MainActor
 final class PreinstallingMock: Preinstalling {
+    
+    // MARK: Properties
 
     /// The stream of events the service emits while preinstalling.
     let events: AsyncStream<PreinstallingEvent>
@@ -20,10 +22,14 @@ final class PreinstallingMock: Preinstalling {
 
     /// The continuation that feeds ``events``.
     private let continuation: AsyncStream<PreinstallingEvent>.Continuation
+    
+    // MARK: Initializers
 
     init() {
         (events, continuation) = AsyncStream.makeStream(of: PreinstallingEvent.self)
     }
+    
+    // MARK: Methods
 
     /// Emits the given event through ``events``.
     ///