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.cpp

@@ -123,6 +123,18 @@ TrackPlan makeRestorePlan(const std::string& guid, const TrackSnapshot& snap) {
     return p;
 }
 
+std::string nextModeId(const ModeRegistry& modes, const std::string& currentModeId) {
+    const std::vector<Mode>& all = modes.all();
+    if (all.empty()) return {};  // nothing to cycle to
+    for (std::size_t i = 0; i < all.size(); ++i) {
+        if (all[i].id == currentModeId)
+            return all[(i + 1) % all.size()].id;  // wrap past the last
+    }
+    // Active mode not in the registry (stale/unknown) — jump to the first mode as a
+    // sane home rather than returning "".
+    return all.front().id;
+}
+
 // ---------------------------------------------------------------------------
 // ViewModeModel
 // ---------------------------------------------------------------------------