iPad: adaptive sidebar+detail shell and Mac-style diff (remote Phases 1-2)

Grow the universal NucleicRemote app into a width-adaptive shell so a
regular-width iPad renders the macOS sidebar+detail IA while the iPhone
keeps its TabView -- both over the same RemoteStore projection (one host
authority, N renderers).

Phase 1 (adaptive shell):
- AdaptiveRootView branches on horizontalSizeClass: CompactRootView (the
  existing iPhone TabView, moved verbatim) vs SplitRootView
  (NavigationSplitView) on regular width.
- SplitSidebar: Home/Projects/To-dos/Settings destinations + sessions
  grouped under their projects (attention-sorted), connection chip footer.
- SplitDetail selects a destination or a session; a selected session reuses
  SessionDetailView keyed .id(sessionID) so switching drives open/close.
- RemoteStore.closeOpen(_:) is now id-guarded so a split-view A->B switch
  (onAppear(B) before onDisappear(A)) can't tear down B's fresh subscription.
- IdentityStore.deviceID idiom-tags the prefix (ipad-/iphone-) for new
  installs so the host lists a paired iPad correctly.

Phase 2 (width tuning + diff):
- readableColumn() caps+centers Home and the transcript on wide layouts;
  a no-op at phone/portrait width.
- SessionDiffView switches on available width (GeometryReader): a Mac-style
  two-pane diff (file list + selected file's patch) on wide/landscape, the
  phone stack otherwise. UnifiedPatch splits the combined patch per file.
  Read-only, same wire, no protocol change.
- Demo diff fixture now carries both files' patches.

iPhone layout and behavior unchanged. Builds clean; verified in the iPad
simulator (demo mode).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

NucleicRemote/NucleicRemote/NucleicRemoteApp.swift

@@ -36,8 +36,6 @@ struct NucleicRemoteApp: App {
 
 struct RootView: View {
     @EnvironmentObject var store: RemoteStore
-    // Initial tab; overridable via NUCLEIC_TAB for offline UI previews.
-    @State private var tab = Int(ProcessInfo.processInfo.environment["NUCLEIC_TAB"] ?? "") ?? 0
     // Appearance preferences — same storage keys and semantics as the Mac's Settings →
     // Appearance, so both devices honor the same choices.
     @AppStorage(AppAppearance.storageKey) private var appearanceRaw = AppAppearance.system.rawValue
@@ -52,30 +50,15 @@ struct RootView: View {
             DemoBanner()
             Group {
                 if store.isPaired {
-                    TabView(selection: $tab) {
-                        HomeView()
-                            .tabItem { Label("Home", systemImage: "house") }.tag(0)
-                        SessionsView()
-                            .tabItem { Label("Sessions", systemImage: "square.stack.3d.up") }
-                            .badge(store.needsYouCount).tag(1)
-                        ProjectsView()
-                            .tabItem { Label("Projects", systemImage: "folder") }.tag(2)
-                        TodosView()
-                            .tabItem { Label("To-dos", systemImage: "checklist") }.tag(3)
-                        SettingsView()
-                            .tabItem { Label("Settings", systemImage: "gearshape") }.tag(4)
-                    }
+                    // Width-adaptive shell: the iPhone's TabView on compact width, the Mac's
+                    // sidebar+detail split on a regular-width iPad (see AdaptiveRootView).
+                    AdaptiveRootView()
                 } else {
                     PairingIntroView()
                 }
             }
         }
         .overlay(alignment: .bottom) { ErrorBubble() }
-        // A notification tap routes to its session: jump to the Sessions tab, where
-        // `SessionsView` consumes `pendingRoute` and pushes the detail.
-        .onChange(of: store.pendingRoute) { _, route in
-            if route != nil { tab = 1 }
-        }
         .tint(Palette.accent)
         .preferredColorScheme((AppAppearance(rawValue: appearanceRaw) ?? .system).colorScheme)
         .dynamicTypeSize((AppTextSize(rawValue: textSizeRaw) ?? .medium).dynamicTypeSize)