feat(ipad): W1-W3 — adaptive split-view layout + finding fixes

T-iPad-2: AdaptiveRootView/LayoutPolicy (sole size-class decision), SplitRootView
(NavigationSplitView sidebar+detail), StackRootView (iPhone path verbatim, zero
regression); privacy shade hoisted to shared ZStack top for both branches
T-iPad-3: KeyBarVisibility predicate (hide when hardware keyboard present),
pointer context menu (copy/new-in-cwd/kill, all via existing channels)
T-iPad-4: Projects multi-column grid on iPad (idiom-gated), adaptive sheet detents
T-iPad-5 findings (4/4 fixed): kill onKillSession thread-through +
AppCoordinator.killCurrentSession; split route-gated to .sessions (iPad first-run
pairing); continue-last banner in split sidebar; iPad XCUITest deferred (covered
by SidebarSelectionTests)
Verified: iPhone 16 277 + iPad Pro 11 278 tests green; packages 261 + integration 10;
zero changes under ios/Packages, src/, public/

ios/App/WebTerm/Wiring/LayoutMode.swift

@@ -0,0 +1,22 @@
+import SwiftUI
+
+/// T-iPad-2 · 自适应根视图的布局模式。
+public enum LayoutMode: Equatable {
+    /// 现有 iPhone 路径：`NavigationStack`（列表 → push 终端）。
+    case stack
+    /// iPad 分栏：`NavigationSplitView`（sidebar 会话列表 + detail 终端）。
+    case split
+}
+
+/// T-iPad-2 · **唯一** size-class 决策点（仿 `PrivacyShadePolicy` 的纯谓词
+/// 先例）。视图内严禁散落 `if sizeClass == …`（PLAN_IOS_IPAD §4）—— 所有
+/// 分支只从这里读一次，故可 100% 单测。
+///
+/// regular 宽度（iPad 全屏 / 大分屏 / Stage Manager 大窗）→ `.split`；
+/// compact 或 nil（iPhone / iPad Slide Over / 小分屏 / size class 未定）→
+/// `.stack`。nil 走最保守回退：退到现有已测的 iPhone 路径，绝不误判成分栏。
+public enum LayoutPolicy {
+    public static func mode(horizontalSizeClass: UserInterfaceSizeClass?) -> LayoutMode {
+        horizontalSizeClass == .regular ? .split : .stack
+    }
+}