Collapse a Design View mode switch and its background FX parking to ONE undo point

applyMode now returns with its undo block open; the reclaimer closes it when the drain
settles. A transport deferral or a tab switch ends it early -- two points, never a leak.

src/app/main.cpp

@@ -324,6 +324,10 @@ extern "C" REAPER_PLUGIN_DLL_EXPORT int REAPER_PLUGIN_ENTRYPOINT(
             // altered transport/cursor in the user's project on unload.
             capture::AbortRealtimeCaptureForUnload(g_session);
 
+            // Same reason, for the reclaimer's undo span: a block left open here has no
+            // owner left to close it and would swallow every edit the user makes after.
+            reasampler::fxReclaimEndUndoSpan();
+
             g_rec->Register("-timer", (void*)&OnTimer);
             g_rec->Register("-projectconfig", (void*)&g_projectConfig);
             g_rec->Register("-toggleaction", (void*)&OnToggleAction);