fx_skid

Multiply-blended surface shader for tyre skid marks (nring.carfx): one hardware-instanced flat quad per mark segment, laid a few centimetres above the asphalt along each sliding tyre's path. Across the tyre it draws soft shoulders and four tread grooves; along it, a little streaky breakup. lane0 = segment sequence number, strength 0..1, seed. Material head (newest sequence) and ring (ring size) fade the oldest fade_share of the ring away as new rubber is laid; rubber is what a full-strength mark multiplies the road colour by.