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/persist.cpp

@@ -207,6 +207,14 @@ ViewModeModel loadViewModel(ReaProject* proj) {
 }  // namespace
 
 void ReaSamplerSession::loadFromProject(void* proj, const std::string& projectDir) {
+    // Raise the load signal for the D4 reapply-on-open glue. loadFromProject is the
+    // single choke point for every load path (prime, project switch/open, forked-
+    // sibling load), so setting it here — and NOT on the Save-As branch, which keeps
+    // the in-memory model as-is — makes the signal fire exactly when a fresh view
+    // model has been installed and its active mode's visibility needs reapplying.
+    // main.cpp drains it via consumeLoadSignal() on the same tick.
+    loadPending_ = true;
+
     // The view model is restored on EVERY load path (peer-symmetry with the bank
     // reset below): switching to a project with no view state must clear stale
     // in-memory state, not inherit the previous project's. D3 restores MODEL STATE
@@ -264,6 +272,12 @@ std::string ensureProjectGuid(void* proj, const std::string& rppPath,
 
 }  // namespace
 
+bool ReaSamplerSession::consumeLoadSignal() {
+    const bool pending = loadPending_;
+    loadPending_ = false;
+    return pending;
+}
+
 void ReaSamplerSession::poll() {
     std::string rppPath;
     void* proj = readActiveProject(rppPath);