fix(actions): persist Design View state after every mutating action

Design View actions mutated the model but never pushed it to ext-state, so
tags/active-mode/snapshots were lost on save/reopen. Add persistViewState()
(saveToActiveProject) after each handler's apply/reapply.

src/actions.cpp

@@ -111,6 +111,15 @@ void reapplyActiveMode() {
     applyMode(g_session->view(), g_session->view().activeModeId(), nullptr);
 }
 
+// Persists both the bank and the Design-View model to the active project's ext
+// state. Called after every state-changing Design View action so the view model
+// is not lost across save/close/reopen. Marking the project dirty is correct —
+// a Design View mutation is a project-level change the user should be prompted
+// to save. Safe when there is no active/saved project (saveToActiveProject no-ops).
+void persistViewState() {
+    g_session->saveToActiveProject();
+}
+
 // -- Action bodies ---------------------------------------------------------
 
 // Toggle: cycle to the next mode in ordinal order (Arrange <-> Design with two
@@ -121,12 +130,14 @@ void doToggleMode() {
         nextModeId(g_session->view().modes(), g_session->view().activeModeId());
     if (target.empty()) return;  // no modes to cycle to (degenerate)
     applyMode(g_session->view(), target, nullptr);
+    persistViewState();
 }
 
 // Direct jump to a named mode. applyMode is a no-op (returns false, no mutation) if
 // the id is unregistered, so an absent mode fails safe.
 void doActivateMode(const std::string& modeId) {
     applyMode(g_session->view(), modeId, nullptr);
+    persistViewState();
 }
 
 // Tag the selection's leaves into `modeId`, then reapply so the change is immediate.
@@ -136,6 +147,7 @@ void doTag(const std::string& modeId) {
     for (const std::string& g : selectedTrackGuids())
         g_session->view().membership().tag(g, modeId);
     reapplyActiveMode();
+    persistViewState();
 }
 
 // Untag the selection entirely (return each to the Arrange default). This is the
@@ -145,6 +157,7 @@ void doUntag() {
     for (const std::string& g : selectedTrackGuids())
         g_session->view().membership().untag(g);
     reapplyActiveMode();
+    persistViewState();
 }
 
 // Toggle the per-track show-both pin for the selection. Read the CURRENT pin of each
@@ -157,6 +170,7 @@ void doShowBoth() {
     for (const std::string& g : selectedTrackGuids())
         m.setShowBoth(g, !m.isShowBoth(g));
     reapplyActiveMode();
+    persistViewState();
 }
 
 }  // namespace