Implement initial song project domain models

Sources/MusicAssistantCore/Services/AI/AIService.swift

@@ -0,0 +1,19 @@
+public protocol AIService: Sendable {
+    func planSongProject(from request: SongPlanningRequest) async throws -> SongPlanningDraft
+}
+
+public struct SongPlanningRequest: Equatable, Sendable {
+    public let userIntent: String
+
+    public init(userIntent: String) {
+        self.userIntent = userIntent
+    }
+}
+
+public struct SongPlanningDraft: Equatable, Sendable {
+    public let summary: String
+
+    public init(summary: String) {
+        self.summary = summary
+    }
+}

Sources/MusicAssistantCore/Services/PromptCompiler/PromptCompiling.swift

@@ -0,0 +1,15 @@
+public protocol PromptCompiling: Sendable {
+    associatedtype Project
+
+    func compile(project: Project) throws -> CompiledSunoOutput
+}
+
+public struct CompiledSunoOutput: Equatable, Sendable {
+    public let lyricsText: String
+    public let stylePrompt: String
+
+    public init(lyricsText: String, stylePrompt: String) {
+        self.lyricsText = lyricsText
+        self.stylePrompt = stylePrompt
+    }
+}