Stream visual scenery in the driver (play starts after infrastructure pass)

Splits the progressive load across modes: the loader runs only the first
(infrastructure) pass and then hands off to the driver, so play begins as soon
as tracks/traction/events/signals/the player train are ready. The deferred
visual nodes (3d models, terrain shapes/lines) then stream in from the driver,
a small budget per frame, so the world fills in without a long up-front wait.

- state_serializer: the infrastructure pass now returns to the caller (instead of
  chaining the visual pass), restarting the twin for the visual pass; the visual
  pass uses a small ~8 ms/frame budget and marks the load done at the end. A
  text/compile load (no twin) stays single-pass and finishes as before.
- state_manager: retains the load state across the loader->driver hand-off and
  exposes loading_visuals()/continue_loading_visuals().
- driver_mode::update(): advances the deferred visual load each frame.

Verified on td.scn (playable in ~2 s, visuals complete a few seconds later, no
duplicate objects) and on the large tomaszewo scenery (flora/terrain/models
deferred while infrastructure loads). Inserts run on the main thread before the
render step, so progressive loading needs no locking.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

simulation/simulation.cpp

@@ -56,7 +56,8 @@ bool is_ready { false };
 std::shared_ptr<deserializer_state>
 state_manager::deserialize_begin(std::string const &Scenariofile) {
 
-	return m_serializer.deserialize_begin( Scenariofile );
+	m_loadstate = m_serializer.deserialize_begin( Scenariofile );
+	return m_loadstate;
 }
 
 bool
@@ -65,6 +66,26 @@ state_manager::deserialize_continue(std::shared_ptr<deserializer_state> state) {
 	return m_serializer.deserialize_continue(state);
 }
 
+// true while a progressive load still has visual nodes to stream in (driver phase)
+bool
+state_manager::loading_visuals() const {
+
+	return ( m_loadstate && m_loadstate->visualphase && ( false == m_loadstate->done ) );
+}
+
+// advances the deferred visual-node load by one budgeted step; clears the state when done.
+// called every frame from the driver so the world's visuals fill in after play starts.
+void
+state_manager::continue_loading_visuals() {
+
+	if( false == loading_visuals() ) { return; }
+	m_serializer.deserialize_continue( m_loadstate );
+	if( m_loadstate->done ) {
+		WriteLog( "Progressive visual load complete" );
+		m_loadstate = nullptr;
+	}
+}
+
 // stores class data in specified file, in legacy (text) format
 void
 state_manager::export_as_text( std::string const &Scenariofile ) const {

simulation/simulation.h

@@ -55,6 +55,12 @@ public:
 	// continues deserialization for given context, amount limited by time, returns true if needs to be called again
 	bool
 	    deserialize_continue(std::shared_ptr<deserializer_state> state);
+	// progressive load: true while deferred visual nodes are still streaming in (driver phase)
+	bool
+	    loading_visuals() const;
+	// advances the deferred visual-node load by one budgeted step (call once per frame)
+	void
+	    continue_loading_visuals();
     // stores class data in specified file, in legacy (text) format
     void
         export_as_text( std::string const &Scenariofile ) const;
@@ -62,6 +68,7 @@ public:
 private:
 // members
     state_serializer m_serializer;
+    std::shared_ptr<deserializer_state> m_loadstate; // retained across loader->driver for visual streaming
     struct {
         std::shared_ptr<TMemCell> weather, time, date;
     } m_scriptinginterface;

simulation/simulationstateserializer.cpp

@@ -148,7 +148,10 @@ state_serializer::deserialize_continue(std::shared_ptr<deserializer_state> state
         }
 
 		auto timenow = std::chrono::steady_clock::now();
-        if( std::chrono::duration_cast<std::chrono::milliseconds>( timenow - timelast ).count() >= 200 ) {
+        // small per-frame budget while streaming visuals in the driver (avoid stutter),
+        // generous budget while the loading screen is up (infrastructure pass)
+        auto const budget = ( state->visualphase ? 8 : 200 );
+        if( std::chrono::duration_cast<std::chrono::milliseconds>( timenow - timelast ).count() >= budget ) {
             Application.set_progress( Input.getProgress(), Input.getFullProgress() );
 			return true;
         }
@@ -161,13 +164,15 @@ state_serializer::deserialize_continue(std::shared_ptr<deserializer_state> state
         deserialize_firstinit( Input, Scratchpad );
     }
 
-    // first (infrastructure) pass finished: run a second pass over the same twin to load
-    // the visual nodes that were skipped. only possible when replaying a binary twin; a
-    // text/compile load did everything in one pass (restartReplay returns false).
+    // first (infrastructure) pass finished: the scenario is now playable (tracks, events,
+    // signals, the player train are all loaded). hand control back so the loader can switch
+    // to the driver; the visual nodes load progressively from the driver via a second pass
+    // over the same twin. only possible when replaying a binary twin -- a text/compile load
+    // did everything in one pass (restartReplay returns false).
     if( ( false == state->visualphase )
      && ( true == Input.restartReplay( scene::scenery_load_pass::visual ) ) ) {
         state->visualphase = true;
-        return true; // continue with the visual pass
+        return false; // infrastructure ready -> go to driver; visuals continue there
     }
 
     scene::Groups.close();
@@ -182,6 +187,7 @@ state_serializer::deserialize_continue(std::shared_ptr<deserializer_state> state
 	// before we report the scenario as loaded
 	scene::scenerybinary_wait_all();
 
+	state->done = true;
 	return false;
 }
 

simulation/simulationstateserializer.h

@@ -25,6 +25,8 @@ struct deserializer_state {
 	// progressive (two-pass) load over a binary twin: first pass loads infrastructure,
 	// second pass loads visual nodes. false while in the first (infrastructure) pass.
 	bool visualphase { false };
+	// set once the whole load (both passes / single text pass) has fully finished
+	bool done { false };
 
 	deserializer_state(std::string const &File, cParser::buffertype const Type, const std::string &Path, bool const Loadtraction)
 	    : scenariofile(File), input(File, Type, Path, Loadtraction) { }