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/view_mode_model.h

@@ -313,4 +313,15 @@ TrackPlan makeParkPlan(const std::string& guid, int fxCount);
 // restore-contract invariant directly.
 TrackPlan makeRestorePlan(const std::string& guid, const TrackSnapshot& snap);
 
+// The next mode id in the registry's ordinal order, cycling past `currentModeId`
+// and wrapping to the first mode after the last (Arrange -> Design -> Arrange with
+// the two seed modes; the same cycle scales to N modes with no call-site change).
+// This is the pure decision behind the "toggle active mode" action: the shell reads
+// the model's active mode, asks for the next one, and applies it.
+//   * empty registry           -> "" (nothing to cycle to)
+//   * currentModeId not present -> the first mode's id (a sane home to jump to)
+// Exposed as a free function (not a model member) so it is unit-testable against a
+// bare ModeRegistry without a full ViewModeModel.
+std::string nextModeId(const ModeRegistry& modes, const std::string& currentModeId);
+
 } // namespace reasampler