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/WebTermTests/ProjectsLayoutTests.swift

@@ -0,0 +1,75 @@
+import SwiftUI
+import Testing
+import UIKit
+@testable import WebTerm
+
+/// T-iPad-4 · Projects 大屏化的纯布局决策单测。判据是**设备 idiom + 可用宽度**
+/// （单点，仿 `LayoutPolicy` 先例）——见 `ProjectsLayout` 注释解释为何用 idiom
+/// 而非 size class（iPad 的 Projects 表单 sheet 内部恒 compact，与 iPhone 横屏
+/// 无异，size class 无法在不回归 iPhone 的前提下区分二者）。
+///
+/// iPhone 零回归的可测护栏：`.phone` 分支必须与「现状」逐字节等价——列数恒
+/// 为 1（现有单列 List）、不走网格、sheet detents 恒为 nil（不加修饰符 = 现有
+/// 默认全高 sheet）。`.pad` 分支才是新增的大屏行为。
+@Suite("ProjectsGridLayout column count (T-iPad-4)")
+struct ProjectsGridLayoutTests {
+    // MARK: - iPhone → 恒 1 列 / 不走网格（字节级不变）
+
+    @Test(".phone → 恒 1 列，宽度无关（含横屏宽尺寸，现有单列 List 原样复用）")
+    func phoneIsAlwaysSingleColumn() {
+        #expect(ProjectsGridLayout.columnCount(availableWidth: 320, idiom: .phone) == 1)
+        // iPhone 横屏很宽（~852）也绝不多列 —— 零回归的关键护栏。
+        #expect(ProjectsGridLayout.columnCount(availableWidth: 852, idiom: .phone) == 1)
+        #expect(ProjectsGridLayout.columnCount(availableWidth: 2000, idiom: .phone) == 1)
+    }
+
+    @Test(".phone 不走网格；.pad 走网格")
+    func usesGridOnlyOnPad() {
+        #expect(ProjectsGridLayout.usesGrid(idiom: .phone) == false)
+        #expect(ProjectsGridLayout.usesGrid(idiom: .pad) == true)
+    }
+
+    // MARK: - iPad → 按可用宽度 1/2/3 列
+
+    @Test(".pad 且窄于两列阈值 → 1 列（极窄 iPad 多任务，绝不挤压）")
+    func padNarrowIsSingleColumn() {
+        #expect(ProjectsGridLayout.columnCount(
+            availableWidth: ProjectsGridLayout.twoColumnMinWidth - 1, idiom: .pad) == 1)
+    }
+
+    @Test(".pad 且在 [两列阈值, 三列阈值) → 2 列（表单 sheet 内宽即此档）")
+    func padMediumIsTwoColumns() {
+        #expect(ProjectsGridLayout.columnCount(
+            availableWidth: ProjectsGridLayout.twoColumnMinWidth, idiom: .pad) == 2)
+        #expect(ProjectsGridLayout.columnCount(
+            availableWidth: ProjectsGridLayout.threeColumnMinWidth - 1, idiom: .pad) == 2)
+    }
+
+    @Test(".pad 且 >= 三列阈值 → 3 列（更宽面板/横屏全宽）")
+    func padWideIsThreeColumns() {
+        #expect(ProjectsGridLayout.columnCount(
+            availableWidth: ProjectsGridLayout.threeColumnMinWidth, idiom: .pad) == 3)
+        #expect(ProjectsGridLayout.columnCount(availableWidth: 1400, idiom: .pad) == 3)
+    }
+
+    @Test("列数恒 >= 1（任何宽度都不返回 0/负，防空网格崩溃）")
+    func columnCountIsAlwaysPositive() {
+        for width in stride(from: CGFloat(0), through: 2000, by: 137) {
+            #expect(ProjectsGridLayout.columnCount(availableWidth: width, idiom: .pad) >= 1)
+            #expect(ProjectsGridLayout.columnCount(availableWidth: width, idiom: .phone) == 1)
+        }
+    }
+}
+
+@Suite("ProjectsSheetSizing (T-iPad-4)")
+struct ProjectsSheetSizingTests {
+    @Test(".phone → nil（不加 detent 修饰符 = 现有默认全高 sheet，字节级不变）")
+    func phoneIsNil() {
+        #expect(ProjectsSheetSizing.detents(idiom: .phone) == nil)
+    }
+
+    @Test(".pad → [.medium, .large]（iPad 卡片式，不铺满大屏；仍可上拉到大）")
+    func padIsCardDetents() {
+        #expect(ProjectsSheetSizing.detents(idiom: .pad) == [.medium, .large])
+    }
+}