feat(s3): SwiftUI sage 3-pane shell + 6 pages + AI seam
- AppFeature: SageTheme tokens, AppModel (@MainActor @Observable store), RootView (DEVONthink NavigationSplitView), Dashboard/Documents(MD-first+pending fallback+?token= download)/Search/Ask/Memos/Digest pages
- AI seam: AIService actor + AIResult, AppAIComposition (MockAIProvider x4 tiers), AICompletionView (numbered citations + always-visible routing badge), backend picker with visible explicit-unavailable error
- MarkdownView: block-aware renderer (GFM table separator-row skip, AttributedString inline-only)
- DSApp: thin @main, injects FixtureDSClient + mock AIRouter (zero backend / zero LLM)
swift build (full app) + swift test (19) green under Swift 6 strict concurrency. Sources/AI untouched (isolation vs freeze 17f8830 = clean).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Hyungi
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