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>

Sources/AppFeature/Shell/Components.swift

@@ -0,0 +1,71 @@
+import SwiftUI
+
+struct StatCard: View {
+    let title: String
+    let value: Int
+    let color: Color
+    var body: some View {
+        VStack(alignment: .leading, spacing: 4) {
+            Text(title).font(.caption).foregroundStyle(Sage.muted)
+            Text("\(value)").font(.title.weight(.bold)).foregroundStyle(color)
+        }
+        .frame(maxWidth: .infinity, alignment: .leading)
+        .padding(14)
+        .background(Sage.card, in: RoundedRectangle(cornerRadius: 12))
+        .overlay(RoundedRectangle(cornerRadius: 12).stroke(Sage.line))
+    }
+}
+
+struct DomainBar: View {
+    let name: String
+    let count: Int
+    let max: Int
+    var body: some View {
+        HStack(spacing: 10) {
+            Text(name).font(.caption).foregroundStyle(Sage.ink)
+                .frame(width: 120, alignment: .leading).lineLimit(1)
+            GeometryReader { geo in
+                ZStack(alignment: .leading) {
+                    RoundedRectangle(cornerRadius: 4).fill(Sage.surface)
+                    RoundedRectangle(cornerRadius: 4)
+                        .fill(Sage.domainColor(name))
+                        .frame(width: geo.size.width * fraction)
+                }
+            }
+            .frame(height: 10)
+            Text("\(count)").font(.caption.monospacedDigit()).foregroundStyle(Sage.muted)
+                .frame(width: 44, alignment: .trailing)
+        }
+    }
+    private var fraction: CGFloat { max > 0 ? CGFloat(count) / CGFloat(max) : 0 }
+}
+
+struct ScoreBar: View {
+    let score: Double
+    var body: some View {
+        HStack(spacing: 6) {
+            GeometryReader { geo in
+                ZStack(alignment: .leading) {
+                    RoundedRectangle(cornerRadius: 3).fill(Sage.surface)
+                    RoundedRectangle(cornerRadius: 3).fill(Sage.brand)
+                        .frame(width: geo.size.width * CGFloat(min(max(score, 0), 1)))
+                }
+            }
+            .frame(height: 6)
+            Text(String(format: "%.2f", score)).font(.caption2.monospacedDigit()).foregroundStyle(Sage.muted)
+        }
+    }
+}
+
+struct ErrorBanner: View {
+    let text: String
+    var body: some View {
+        Text(text)
+            .font(.callout)
+            .foregroundStyle(Sage.danger)
+            .frame(maxWidth: .infinity, alignment: .leading)
+            .padding(12)
+            .background(Sage.danger.opacity(0.08), in: RoundedRectangle(cornerRadius: 10))
+            .overlay(RoundedRectangle(cornerRadius: 10).stroke(Sage.danger.opacity(0.3)))
+    }
+}

Sources/AppFeature/Shell/RootView.swift

@@ -0,0 +1,116 @@
+import SwiftUI
+import DSKit
+
+/// DEVONthink-style 3-column shell. RootView only ROUTES; each page owns its own interior treatment
+/// (no shell-level auto-inherit). macOS-only target.
+public struct RootView: View {
+    @Environment(AppModel.self) private var model
+    @State private var columnVisibility: NavigationSplitViewVisibility = .all
+
+    public init() {}
+
+    public var body: some View {
+        NavigationSplitView(columnVisibility: $columnVisibility) {
+            Sidebar()
+                .navigationSplitViewColumnWidth(min: 220, ideal: 250)
+        } content: {
+            ContentColumn()
+                .navigationSplitViewColumnWidth(min: 300, ideal: 380)
+        } detail: {
+            DetailColumn()
+        }
+        .navigationSplitViewStyle(.balanced)
+        .tint(Sage.brand)
+        .task { await model.loadInitial() }
+    }
+}
+
+struct Sidebar: View {
+    @Environment(AppModel.self) private var model
+
+    var body: some View {
+        let selection = Binding<AppModel.Section?>(
+            get: { model.section },
+            set: { if let v = $0 { model.section = v } }
+        )
+        List(selection: selection) {
+            Section {
+                ForEach(AppModel.Section.allCases) { s in
+                    Text(s.title).tag(s)
+                }
+            }
+            if model.section == .documents, !model.tree.isEmpty {
+                Section("도메인") {
+                    ForEach(model.tree) { node in
+                        DomainRow(node: node)
+                    }
+                }
+            }
+        }
+        .listStyle(.sidebar)
+        .background(Sage.sidebar)
+    }
+}
+
+struct DomainRow: View {
+    @Environment(AppModel.self) private var model
+    let node: DomainTreeNode
+
+    var body: some View {
+        HStack(spacing: 8) {
+            Circle().fill(Sage.domainColor(node.name)).frame(width: 8, height: 8)
+            Text(node.name).font(.callout).foregroundStyle(Sage.ink)
+            Spacer()
+            Text("\(node.count)").font(.caption).foregroundStyle(Sage.muted)
+        }
+        .contentShape(Rectangle())
+        .onTapGesture { model.section = .documents }
+    }
+}
+
+struct ContentColumn: View {
+    @Environment(AppModel.self) private var model
+
+    var body: some View {
+        Group {
+            switch model.section {
+            case .dashboard: DashboardView()
+            case .documents: DocumentListView()
+            case .search: SearchView()
+            case .ask: AskView()
+            case .memos: MemoListView()
+            case .digest: DigestView()
+            }
+        }
+        .navigationTitle(model.section.title)
+    }
+}
+
+struct DetailColumn: View {
+    @Environment(AppModel.self) private var model
+
+    var body: some View {
+        Group {
+            switch model.section {
+            case .documents:
+                if let d = model.documentDetail { DocumentDetailView(detail: d) }
+                else { EmptyState(text: "문서를 선택하세요") }
+            case .memos:
+                if let m = model.memoDetail { MemoDetailView(memo: m) }
+                else { EmptyState(text: "메모를 선택하세요") }
+            default:
+                EmptyState(text: model.section.title)
+            }
+        }
+    }
+}
+
+struct EmptyState: View {
+    let text: String
+    var body: some View {
+        Text(text)
+            .foregroundStyle(Sage.muted)
+            .frame(maxWidth: .infinity, maxHeight: .infinity)
+            .background(Sage.surface)
+    }
+}