Add temporary undo-span tracing for the mid-drain Ctrl-Z bug

One switch (REASAMPLER_UNDO_TRACE) gates every line. Traces span open/close
reasons, FX writes, UndoBlock depth, timer standoffs, and load-state hooks.

src/shell/undo/undo_block.cpp

@@ -5,6 +5,7 @@
 
 #include <utility>
 
+#include "shell/undo/undo_trace.h"  // TEMPORARY — see undo_trace.h; revert together
 #include "shell/view/fx_reclaim.h"  // fxReclaimEndUndoSpan — the span this must not nest over
 
 #define REAPERAPI_MINIMAL
@@ -40,12 +41,13 @@ UndoBlock::UndoBlock(ReaProject* proj) : project_(proj) {
     // outermost. Roll the depth back on any throw so the type degrades to "this one
     // block failed to open," not "every block after it is now poisoned."
     try {
-        if (g_depth++ == 0) fxReclaimEndUndoSpan();
+        if (g_depth++ == 0) fxReclaimEndUndoSpan("UndoBlock ctor");
         Undo_BeginBlock2(proj);
     } catch (...) {
         --g_depth;
         throw;
     }
+    REASAMPLER_TRACE("[UndoBlock] CTOR proj=%s depth=%d", proj ? "real" : "active", g_depth);
 }
 
 UndoBlock::~UndoBlock() {
@@ -63,6 +65,8 @@ UndoBlock::~UndoBlock() {
     if (!project_ || ValidatePtr2(nullptr, project_, "ReaProject*"))
         Undo_EndBlock2(project_, label_.c_str(), mask_);
     --g_depth;
+    REASAMPLER_TRACE("[UndoBlock] DTOR label=\"%s\" mask=%d depth=%d",
+                      label_.empty() ? "(discard)" : label_.c_str(), mask_, g_depth);
 }
 
 void UndoBlock::recordPoint(std::string label, int mask) {