racingline_mat

# racingline Alpha-blended, self-lit surface shader for the Forza-style driving line (`nring.racingline`). It draws one whole-lap ribbon mesh laid a few centimetres above the asphalt: `uv.x` runs across the strip, `uv.y` is the vertex's lap distance (m), and the vertex colour carries the target speed (`r` × 100 m/s), the corner-zone weight (`g`) and the action (`b`: 0 accelerate, 0.5 lift, 1 brake). Instance lane 0 is the car's lap distance, its speed, the lap length and the corners-only flag; lane 1 is the reach ahead, the fade behind, the opacity and a debug switch. Only the stretch from a few metres behind the car to `reach` ahead draws, coloured green / yellow / red by how far the car's current speed is above the target speed at each fragment (`lift_share`, `brake_share`), with soft edges and forward chevrons that fade out with distance.