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>
