Use the original computer drivers, car collisions and towing

Decoded from SKIDMARKS_CD32:

- Computer drivers: the path of nodes laid 2 game units apart along
  the middle of the road, with side points 1 unit in from each wall.
  Drivers aim four nodes past where they'll be in four frames, or cut
  to the other side two nodes on at 70% throttle when running wide.
  Replaces the pure-pursuit AI; laps are quicker on every track.
- Collisions: six points on the player's car (corners and sides, from
  .set words 6 and 7, now known to be half-length and half-width) are
  tested against the other car's box. Hits push the cars apart with a
  strength that grows with their speed difference, and spin the car.
  As in the original, only human players' cars collide; cars at
  different heights don't.
- Towing: the hitch is at the car's back, the caravan sits 0.656 units
  behind it, and past 90 degrees both car and caravan bend back.

The reconstructed collision weight (`mass`) is gone; old asset files
with it still load.

README.md

@@ -44,7 +44,8 @@ give either to also gets the game data.
 - The original title picture.
 - 1 or 2 players (split screen) plus up to 7 computer drivers.
 - Power-slides, jumps, hills that slow you going up and speed you up coming
-  down, car-to-car bumping, and skid marks burnt into the track.
+  down, spinning bumps against other cars, and skid marks burnt into the
+  track.
 - Lap and position tracking, lap times, and a results table.
 - Engine note pitched with revs, tyre screech, crashes, backfires, a horn
   and start beeps, all from the original samples.
@@ -57,12 +58,14 @@ were decoded from its machine code and work the same way here:
 - the meaning of every `.set` value, and the handling presets
 - the walls along the road edges
 - how the world is projected onto the screen, including height
+- the computer drivers' path and how they choose their target and throttle
+- collisions between cars. As in the original, only human players' cars
+  collide: computer cars pass through each other and through caravans.
+- towing, including caravans jack-knifing
 
 The rest is new code written to feel like the original:
-- how computer drivers choose their line (they steer and drive as the
-  original's do)
-- car-to-car collisions
-- lap counting and the rescue of stuck cars
+- lap counting
+- the rescue of stuck cars (the original warps them back instead)
 
 [docs/FORMATS.md](docs/FORMATS.md) covers everything reverse engineered so
 far.