feat(view): D4 Design View action family + reapply-on-open wiring

Register toggle/activate/tag/untag/show-both actions driving the D2
shell and D3 model; reapply saved active mode on project load via a
persist load-signal seam. Shared guidString helper; pure nextModeId
unit-tested.

src/track_guid.cpp

@@ -0,0 +1,24 @@
+// track_guid.cpp — the single MediaTrack* -> canonical GUID key formatter. See
+// track_guid.h. Compiled into the reaper_reasampler MODULE; includes
+// reaper_plugin_functions.h WITHOUT REAPERAPI_IMPLEMENT (main.cpp is the one TU
+// that defines the API pointers — CLAUDE.md §contract).
+
+#include "track_guid.h"
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_GetTrackGUID
+#define REAPERAPI_WANT_guidToString
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+std::string guidString(MediaTrack* tr) {
+    if (!tr) return {};
+    GUID* g = GetTrackGUID(tr);
+    if (!g) return {};
+    char buf[64] = {0};  // guidToString needs a >=64-char destination (SDK contract)
+    guidToString(g, buf);
+    return std::string(buf);
+}
+
+}  // namespace reasampler