Defer Design View's per-FX park to an idle tick; honest undo mask, compare-before-write, PreventUIRefresh bracket, O(1) handle resolve

src/app/CMakeLists.txt

@@ -38,6 +38,7 @@ add_library(reaper_reasampler MODULE
     ${LICE_SRC}
     ${REASAMPLER_SRC_DIR}/shell/capture/insert.cpp
     ${REASAMPLER_SRC_DIR}/shell/view/view.cpp
+    ${REASAMPLER_SRC_DIR}/shell/view/view_fx_park.cpp
     ${REASAMPLER_SRC_DIR}/shell/view/view_solo.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/track_guid.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/provenance_shell.cpp
@@ -59,6 +60,12 @@ target_link_libraries(reaper_reasampler PRIVATE json wire file_bytes bank_model
 # link graph free of the voice engine — a link edge to it here means the design drifted.
 target_include_directories(reaper_reasampler PRIVATE ${SDK_INC} ${WDL_INC})
 
+# The deferred FX-park queue's re-entrancy rule is pure (header-inline, no REAPER
+# types), so it is CTest-covered like a core/ module. Declared here rather than in a
+# src/shell/view/CMakeLists.txt because that directory deliberately has none — its
+# TUs are compiled into this target directly.
+reasampler_test(view_fx_park LINK fx_offline)
+
 # Bank-package import: the promptless verb plus its action skin. Kept as its own
 # appended block rather than merged into the lists above, so the two package
 # directions stay textually independent.

src/app/main.cpp

@@ -38,6 +38,7 @@
 #include "shell/panel/panel_window.h"            // panel lifecycle (init/toggle/open-query/shutdown)
 #include "shell/persist/session.h"               // ReaSamplerSession
 #include "shell/view/view.h"                     // reconcileManagedLanes / applyMode
+#include "shell/view/view_fx_park.h"             // the mode switch's deferred FX park
 
 namespace capture = reasampler::capture;
 
@@ -172,6 +173,12 @@ static void OnTimer()
 
     g_session.poll();
 
+    // A mode switch applies its visibility/routing writes synchronously and leaves
+    // the per-FX offline work here, so the new mode paints before the plugins
+    // unload. Drained AFTER poll() so a project switch discards the queue instead
+    // of applying it to the project that replaced it; idle cost is one empty test.
+    reasampler::drainDeferredFxParks(g_session.view());
+
     // persist stays MODEL-ONLY (loads the saved view model but does not apply
     // visibility, to avoid coupling persist to the view shell); poll() raises a
     // one-shot load signal that we drain here to reapply the SAVED active mode so a