refactor(retention): use Framework.RunOnTick instead of synchronous .Wait()

Retention sweep no longer blocks for ~194ms on Framework.Run().Wait().
The clear+refilter pair is now scheduled on the next framework tick, so
it still runs on the framework thread (keeping the Tabs-list mutation
serialisation invariant -- Plugin.Config.Tabs is plain List<Tab> and
AutoTellTabsService can mutate it from background paths) but does not
block the sweep thread while the framework finishes the current frame.

A new _isDisposing volatile bool is set as the first statement in
DisposeAsync so a deferred tick that fires after teardown bails before
it touches MessageManager / Log / static fields the dispose path has
already cleared. The retention worker is IsBackground=true so plugin
unload can race against a still-pending tick.

The existing RetentionSweepLock / RetentionSweepRunning serialisation
covers the not-two-sweeps-at-once invariant; we don't add a CTS here
because RunOnTick is fire-and-forget and the framework service owns
the tick lifecycle.

v1.4.8 B3. Coverage via in-game smoke (frame-time trace during a
retention sweep run) in Task 9 -- no Build-Suite test because the
suite has no FakeFramework fixture and the change is a schedule-form
swap rather than new behaviour.

HellionChat/Plugin.cs

@@ -126,6 +126,12 @@ public sealed class Plugin : IAsyncDalamudPlugin
     // Idempotency guard — Dalamud may fire DisposeAsync twice in a reload race.
     private int _disposeStarted;
 
+    // Set in the first DisposeAsync statement so async callbacks scheduled
+    // via Framework.RunOnTick (v1.4.8 B3 retention sweep) can early-bail
+    // before they touch state that has already been torn down. Volatile
+    // because the tick reads it from a different thread than the writer.
+    private volatile bool _isDisposing;
+
     internal int DeferredSaveFrames = -1;
 
     // Cancels the v1.4.8 FTS5 bulk-insert worker on plugin teardown. The
@@ -440,6 +446,12 @@ public sealed class Plugin : IAsyncDalamudPlugin
         if (Interlocked.Exchange(ref _disposeStarted, 1) != 0)
             return;
 
+        // Set before any cleanup so deferred Framework.RunOnTick callbacks
+        // (B3 retention sweep) see the flag and bail out before they touch
+        // MessageManager / Log / static fields that the rest of this method
+        // is about to tear down.
+        _isDisposing = true;
+
         Exception? failure = null;
 
         // Unsubscribe hooks first — mirrors the hooks-last subscribe order in LoadAsync.
@@ -711,15 +723,31 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 if (deleted > 0)
                 {
                     Log.Information($"Retention sweep deleted {deleted} expired messages.");
-                    // Run clear+refilter on the framework thread — FilterAllTabsAsync
+                    // Schedule on the next framework tick to avoid the ~194ms
-                    // is fire-and-forget and would race the next sweep cycle.
+                    // hitch from blocking with .Wait() while the framework
-                    Framework
+                    // finishes the current frame. Tabs-list mutation must
-                        .Run(() =>
+                    // stay on the framework thread because Plugin.Config.Tabs
+                    // (Configuration.cs:222) is not lock-protected and
+                    // AutoTellTabsService can mutate it from background paths.
+                    // Pattern reference: SimpleTweaks
+                    // Tweaks/Chat/CaseInsensitiveCommands.cs:45.
+                    Framework.RunOnTick(() =>
+                    {
+                        // The retention thread is IsBackground=true so plugin
+                        // unload can fire while a scheduled tick is still
+                        // pending; bail before touching anything torn down.
+                        if (_isDisposing)
+                            return;
+                        try
                         {
                             MessageManager.ClearAllTabs();
                             MessageManager.FilterAllTabs();
-                        })
+                        }
-                        .Wait();
+                        catch (Exception ex)
+                        {
+                            Log.Error(ex, "Retention sweep clear+refilter failed");
+                        }
+                    });
                 }
                 else
                 {