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.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;