merge: integrate AIFabric (S2) into S3 app — unified package

- Resolve Package.swift add/add: one manifest, single AIFabric target (Sources/AI compiled once;
  no duplicate-symbol risk) + DSKit/AppFeature/DSApp + AITests + DSKitTests, AIFabric library product kept.
- import AI -> import AIFabric across AppFeature + RouterFallbackTests (S2 renamed module).
- AppModel.askMeta qualified DSKit.AskResponse (AIFabric also defines an AskResponse for RemoteDS).

swift build + swift test green (71 tests: S2 AITests + S3 DSKitTests). Frozen AIProvider interface intact.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Sources/AppFeature/AI/AICompletionView.swift

@@ -1,5 +1,5 @@
 import SwiftUI
-import AI
+import AIFabric
 
 public extension AIProviderID {
     var displayName: String {

Sources/AppFeature/AI/AIService.swift

@@ -1,5 +1,5 @@
 import Foundation
-import AI
+import AIFabric
 
 /// Renderable failure (the UI never sees a raw AIRoutingError — that would break the
 /// "visible error, not silent fallback" contract).

Sources/AppFeature/AI/AppAIComposition.swift

@@ -1,5 +1,5 @@
 import Foundation
-import AI
+import AIFabric
 
 /// The ONE composition touch-point that names MockAIProvider. When S2 ships real providers,
 /// only this file changes (mockProviders -> realProviders) — AIService, views, and intents stay put.

Sources/AppFeature/Pages/AskView.swift

@@ -1,5 +1,5 @@
 import SwiftUI
-import AI
+import AIFabric
 
 /// RAG proof page: routes corpusAsk through AIService (-> AIRouter -> MockAIProvider). Explicit backend
 /// pick sets explicitProvider; an explicit-unavailable result renders a visible, non-retrying error.

Sources/AppFeature/State/AppModel.swift

@@ -1,7 +1,7 @@
 import SwiftUI
 import Observation
 import DSKit
-import AI
+import AIFabric
 
 /// The single app-state store driving the 3-pane shell. @MainActor @Observable: mutations are
 /// main-isolated; the DSClient returns Sendable models; AIService is an actor.
@@ -35,7 +35,7 @@ public final class AppModel {
     public var searchResponse: SearchResponse?
     public var askQuery: String = ""
     public var askResult: AIResult?
-    public var askMeta: AskResponse?
+    public var askMeta: DSKit.AskResponse? // qualified: AIFabric also defines an AskResponse
     public var memoList: [MemoResponse] = []
     public var memoDetail: MemoResponse?
     public var digest: DigestResponse?