feat(main-window): track per-frame window pos/size/viewport for PayloadHandler

Replaces the v1.5.6 `LogWindow.LastViewport/LastWindowPos/LastWindowSize`
window-instance state with public/internal MainWindow surfaces refreshed
at the top of Draw() each frame. PayloadHandler.MoveTooltip in Phase 2
will read these to filter cross-viewport AddonLifecycle events and to
reposition the native item tooltip away from the chat window.

LastViewport is `internal unsafe` (not public) — the only consumer is
PayloadHandler.MoveTooltip in the same assembly; keeping the raw pointer
out of the public surface is the safer default.

Split from Sub-Task A — Lender injection lives in A2 (after F's DI-reg).

HellionChat/Ui/Windows/MainWindow.cs

@@ -29,6 +29,10 @@ internal sealed class MainWindow : Window
 
     private Tab? _activeTab;
 
+    public Vector2 LastWindowPos { get; private set; } = Vector2.Zero;
+    public Vector2 LastWindowSize { get; private set; } = Vector2.Zero;
+    internal unsafe ImGuiViewport* LastViewport;
+
     public MainWindow(
         Components.HonorificHeader honorific,
         Components.Sidebar sidebar,
@@ -81,6 +85,13 @@ internal sealed class MainWindow : Window
 
     public override void Draw()
     {
+        LastWindowPos = ImGui.GetWindowPos();
+        LastWindowSize = ImGui.GetWindowSize();
+        unsafe
+        {
+            LastViewport = ImGui.GetWindowViewport().Handle;
+        }
+
         // First-frame seed: the active tab defaults to the first persisted
         // tab so the message list isn't empty on a clean session.
         if (_activeTab is null && Plugin.Config.Tabs.Count > 0)