fix #390: retail display-change UI cascade — clamp + per-res reload

Decomp-first per the block's rule: the research doc
(docs/research/2026-08-13-retail-ui-display-change.md, committed here)
pulled retail's actual mechanism before any code. A display change runs
UIElementManager::RefreshEvent @0x0045C530 ->
UIElement::UpdateForParentSizeChange @0x00462640, which unconditionally
re-applies every floating window's own clamping MoveTo override
(x = max(0, min(x, parentW - selfW)) - top-left priority, oversized
windows pin to 0), then broadcasts global message 0xE whose sole
listener reloads the per-resolution auto layout. No proportional moves,
no resets; retail saves layouts only via @saveui.

Port: RetailWindowLayoutPersistence.ClampAllToScreen() is the cascade
clamp (no store I/O; _restoring suppresses the per-move save so a live
drag-resize cannot write settings.json per frame), and
RetailUiRuntime.Draw carries a two-step screen-size edge detector:
change frame -> clamp; first stable frame -> one
RestoreAll(saveBack:false) per-resolution reload (the 0xE analog; no
lazy save-back, matching retail's save-only-on-command). The login
restore path already used retail's exact clamp math (Apply) - the live
trigger was the missing half, which is precisely the stranding the user
reported.

Deliberate deviation, register AD-91: retail's gmFloatyChatUI windows
have NO clamp and can strand; the block's requirement ("UI windows must
stay reachable") clamps every registered window uniformly.

Tests: 5 new persistence facts (clamp/top-left-pin/no-move/no-save-on-
clamp/no-save-on-live-reload). App suite 4,967/3 skips. Gate script
section D3 filled in.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/RetailWindowLayoutPersistenceTests.cs

@@ -247,6 +247,111 @@ public sealed class RetailWindowLayoutPersistenceTests : IDisposable
         Assert.False(radar.IsVisible);
     }
 
+    // ── #390: retail's display-change anti-stranding cascade ────────────
+    // The clamp rule is each floating window's own MoveTo override,
+    // x = max(0, min(x, parentW − selfW)) (top-left priority), re-applied
+    // unconditionally on every display change — see
+    // docs/research/2026-08-13-retail-ui-display-change.md.
+
+    [Fact]
+    public void ClampAllToScreen_ReclampsStrandedWindows_TopLeftPriority()
+    {
+        var store = new SettingsStore(PathName);
+        var root = new UiRoot { Width = 2560, Height = 1440 };
+        RetailWindowHandle handle = Mount(
+            root, WindowNames.Examination, width: 310f, height: 400f);
+        handle.MoveTo(2200f, 1000f);   // parked near the bottom-right corner
+        (int Width, int Height) screen = (2560, 1440);
+        using var persistence = new RetailWindowLayoutPersistence(
+            root.WindowManager, store, () => "Alice", () => screen);
+
+        screen = (1280, 720);          // the display shrank
+        persistence.ClampAllToScreen();
+
+        Assert.Equal(1280f - 310f, handle.Left);
+        Assert.Equal(720f - 400f, handle.Top);
+    }
+
+    [Fact]
+    public void ClampAllToScreen_OversizedWindow_PinsTopLeftToZero()
+    {
+        // max(0, min(...)) — the max applies LAST, so a window larger than
+        // the screen pins to 0 and its top-left chrome stays reachable.
+        var store = new SettingsStore(PathName);
+        var root = new UiRoot { Width = 800, Height = 600 };
+        RetailWindowHandle handle = Mount(
+            root, WindowNames.Examination, width: 310f, height: 400f);
+        handle.MoveTo(400f, 300f);
+        (int Width, int Height) screen = (800, 600);
+        using var persistence = new RetailWindowLayoutPersistence(
+            root.WindowManager, store, () => "Alice", () => screen);
+
+        screen = (200, 200);           // smaller than the window itself
+        persistence.ClampAllToScreen();
+
+        Assert.Equal(0f, handle.Left);
+        Assert.Equal(0f, handle.Top);
+    }
+
+    [Fact]
+    public void ClampAllToScreen_InBoundsWindow_DoesNotMove()
+    {
+        var store = new SettingsStore(PathName);
+        var root = new UiRoot { Width = 2560, Height = 1440 };
+        RetailWindowHandle handle = Mount(
+            root, WindowNames.Examination, width: 310f, height: 400f);
+        handle.MoveTo(100f, 120f);
+        using var persistence = new RetailWindowLayoutPersistence(
+            root.WindowManager, store, () => "Alice", () => (1280, 720));
+
+        persistence.ClampAllToScreen();
+
+        Assert.Equal((100f, 120f), (handle.Left, handle.Top));
+    }
+
+    [Fact]
+    public void ClampAllToScreen_DoesNotSaveTheClampedPositions()
+    {
+        // The clamp runs per resize FRAME during a live window drag; its
+        // MoveTo must not trip the per-move save (settings.json writes per
+        // frame, keyed by whatever intermediate resolution the drag passes
+        // through). Only USER moves save.
+        var store = new SettingsStore(PathName);
+        var root = new UiRoot { Width = 2560, Height = 1440 };
+        RetailWindowHandle handle = Mount(
+            root, WindowNames.Examination, width: 310f, height: 400f);
+        handle.MoveTo(2200f, 1000f);   // pre-attach: no save subscription yet
+        (int Width, int Height) screen = (2560, 1440);
+        using var persistence = new RetailWindowLayoutPersistence(
+            root.WindowManager, store, () => "Alice", () => screen);
+
+        screen = (1280, 720);
+        persistence.ClampAllToScreen();
+
+        Assert.Equal(1280f - 310f, handle.Left);   // it DID clamp…
+        Assert.Null(store.LoadWindowLayout(       // …and did NOT write.
+            "Alice", "1280x720", WindowNames.Examination, default));
+    }
+
+    [Fact]
+    public void RestoreAll_WithoutSaveBack_WritesNothingToTheStore()
+    {
+        // The live display-change reload (#390) must not persist — retail
+        // saves layouts only via @saveui/@saveautoui, and a mid-drag reload
+        // writing would litter settings.json with intermediate-resolution
+        // keys.
+        var store = new SettingsStore(PathName);
+        var root = new UiRoot { Width = 800, Height = 600 };
+        _ = Mount(root, WindowNames.Examination, width: 310f, height: 400f);
+        using var persistence = new RetailWindowLayoutPersistence(
+            root.WindowManager, store, () => "Alice", () => (800, 600));
+
+        persistence.RestoreAll(saveBack: false);
+
+        Assert.Null(store.LoadWindowLayout(
+            "Alice", "800x600", WindowNames.Examination, default));
+    }
+
     private static RetailWindowHandle Mount(
         UiRoot root,
         string name,