D3: persist ViewModeModel in project ext state alongside bank

Add forever-stable "view_state" key under the "reasampler" namespace.
saveToActiveProject additionally serializes the ViewModeModel;
loadFromProject restores it on every load path, defaulting on an
absent/malformed key. Model state only — no visibility apply (D4).

src/persist.h

@@ -20,6 +20,7 @@
 #include <string>
 
 #include "bank_model.h"
+#include "view_mode_model.h"
 
 namespace reasampler {
 
@@ -31,6 +32,12 @@ inline constexpr const char* kProjExtNamespace = "reasampler";
 // serialized BankIndex). FOREVER-STABLE for the same reason.
 inline constexpr const char* kProjExtIndexKey = "bank_index";
 
+// The ext-state key the Design-View ViewModeModel JSON is stored under (one key
+// holds the whole serialized model: modes + membership + show-both + snapshots +
+// active mode). Distinct from kProjExtIndexKey — one namespace, two keys.
+// FOREVER-STABLE: changing it orphans every already-saved project's view state.
+inline constexpr const char* kProjExtViewKey = "view_state";
+
 // The ext-state key holding a GUID we mint per project to establish CONTENT-BASED
 // project identity (REAPER exposes no stable per-project GUID). poll() uses it to
 // tell a genuine Save-As (same GUID, new .rpp path) apart from a project switch
@@ -62,6 +69,13 @@ public:
     BankIndex& bank() { return bank_; }
     const BankIndex& bank() const { return bank_; }
 
+    // The in-memory Design-View model. The view/action layer mutates it (tag,
+    // toggle, snapshot); persist serializes it on save and replaces it on project
+    // load — exactly as it treats the bank. D3 persists MODEL STATE only; applying
+    // visibility/processing (reapply-on-open) is D4's job, not this member's.
+    ViewModeModel& view() { return view_; }
+    const ViewModeModel& view() const { return view_; }
+
     // Serialize the current bank to the active project's ext state (namespace
     // "reasampler"). Non-destructive beyond writing our own ext-state key. Safe
     // to call when there is no active/saved project (it no-ops).
@@ -75,6 +89,11 @@ public:
 private:
     BankIndex bank_;
 
+    // The Design-View model. Default-constructed = Arrange + Design seeded, active
+    // = Arrange; loadFromProject leaves this default when a project has no stored
+    // view_state (older project), so an absent key is graceful, not a crash.
+    ViewModeModel view_;
+
     // The project identity last observed by poll(), used to detect load/Save-As.
     // Identity is the GUID we mint per project (kProjExtGuidKey), NOT the raw
     // ReaProject* pointer — see the class comment for why. The .rpp path is