fix(persist): call MarkProjectDirty after every SetProjExtState write

After capture, saveToActiveProject wrote ext state but left the project
clean, so Ctrl+S was a no-op and the bank was silently lost on close.
Also flushes the Save-As re-GUID on the normal save flow.

src/persist.cpp

@@ -54,6 +54,7 @@
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_EnumProjects
 #define REAPERAPI_WANT_GetProjExtState
+#define REAPERAPI_WANT_MarkProjectDirty
 #define REAPERAPI_WANT_SetProjExtState
 #define REAPERAPI_WANT_ShowConsoleMsg
 #define REAPERAPI_WANT_genGuid
@@ -162,6 +163,7 @@ void ReaSamplerSession::saveToActiveProject() {
     const std::string json = bank_.serialize();
     SetProjExtState(static_cast<ReaProject*>(proj), kProjExtNamespace,
                     kProjExtIndexKey, json.c_str());
+    MarkProjectDirty(static_cast<ReaProject*>(proj));
 }
 
 void ReaSamplerSession::loadFromProject(void* proj, const std::string& projectDir) {
@@ -266,11 +268,13 @@ void ReaSamplerSession::poll() {
             // Save-As duplicated our ext state, so the new project B currently
             // shares A's GUID. Mint a FRESH GUID for B and write it, so A and B
             // no longer collide on identity when reopened later. Adopt the fresh
-            // GUID as our last-seen identity.
+            // GUID as our last-seen identity. Mark dirty so the fresh GUID flushes
+            // to the new .rpp on the next normal save / close-prompt.
             const std::string fresh = genProjectGuidString();
             if (proj) {
                 SetProjExtState(static_cast<ReaProject*>(proj), kProjExtNamespace,
                                 kProjExtGuidKey, fresh.c_str());
+                MarkProjectDirty(static_cast<ReaProject*>(proj));
             }
             lastGuid_    = fresh;
             lastRppPath_ = rppPath;