fix(tell): strip stale tell state on tab activation

OnTabActivated clears the runtime tell state the game-side detour leaves on a tab (CurrentChannel tell target + partner label) when a DIFFERENT tab becomes the input surface, so a normal typed line can no longer route as a silent /tell to the old partner. Re-clicking the active tab and tabs carrying their own Tab.TellTarget binding (leg1) are preserved.

All four activation paths route through it: Sidebar, TopTabBar, ChannelPopoutPool.TryOpen, and the MainWindow draw-seed. EnsureCurrentChannel becomes a pure derive-helper reached only via OnTabActivated. Adds the TellResetOnActivateStep self-test (step count 29 -> 30).

HellionChat/Ui/Windows/MainWindow.cs

@@ -156,7 +156,13 @@ internal sealed class MainWindow : Window
         // 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)
-            _activeTab = Plugin.Config.Tabs[0];
+        {
+            var seeded = Plugin.Config.Tabs[0];
+            _activeTab = seeded;
+            // The seeded Tabs[0] is the likeliest legacy stale-tell carrier
+            // (pre-coupling the detour wrote here); strip it like any activation.
+            TabLifecycleHelpers.OnTabActivated(seeded, null);
+        }
 
         var statusHeight = Components.StatusBar.Height;