Train Approach Warning highlights the rail path ahead of moving trains with a configurable glowing overlay.

The warning starts near the leading end of each moving train, follows the train's current path, stays smooth between update ticks, and fades out when the train stops or loses its path. It uses Factorio's rendering API only and does not place entities in the world.

Features:
- Soft glow markers on rails ahead of moving trains
- Configurable update interval and lookahead distance
- Configurable glow radius, opacity, and color
- Optional real light casting from highlighted rails
- Optional over-rail rendering layer
- Smooth persistent rendering between updates
- Fade-out when warnings are removed

This is the initial public release. Feedback on visibility, performance, and edge cases with larger rail networks is welcome.