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.h

@@ -0,0 +1,24 @@
+#pragma once
+// track_guid — the ONE place a MediaTrack* is formatted into the canonical GUID
+// string used as a membership-index key. Both the Design View shell (view.cpp) and
+// the actions layer (actions.cpp) key membership on this exact string, so the key
+// contract lives in a single helper rather than being re-derived (and drifting) at
+// two call sites (the cross-module key contract flagged in D2 review).
+//
+// REAPER-facing: the .cpp includes reaper_plugin_functions.h WITHOUT
+// REAPERAPI_IMPLEMENT (main.cpp owns the API pointers; here they are extern —
+// CLAUDE.md §contract). MediaTrack is forward-declared so this header stays SDK-lite.
+
+#include <string>
+
+class MediaTrack;
+
+namespace reasampler {
+
+// REAPER's canonical "{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}" form of a track's
+// GUID (GetTrackGUID -> guidToString). Empty string if `tr` has no GUID. This IS
+// the membership-index key format — it must match guidToString's braces exactly so
+// the view tree keys and the model/actions keys align.
+std::string guidString(MediaTrack* tr);
+
+}  // namespace reasampler