Convert the game data into editable assets

assets/ now holds every track as a 16-colour indexed PNG (its palette is
the track palette) plus a text file of lap gates and height map, and
every vehicle as a PNG sprite sheet plus a text file with its name and
handling (moved out of the code). The build embeds assets/ instead of
unpacking the ADFs, so the disks are no longer needed to build either.

skids extract converts original disks to assets and checks each file
reads back identically; benchmark results from the assets match the
disks exactly. Assets win over disk data with the same name, and
--data assets picks up edits without a rebuild. assets/README.md
describes how to edit everything.

assets/README.md

@@ -0,0 +1,93 @@
+# Game assets
+
+Everything the game uses is stored here as editable files. The build embeds
+this folder into the executable. To pick up changes:
+- **Rebuild:** `cargo run --release`.
+- **Or skip the rebuild:** run with `--data assets`. Files given with
+  `--data` override the built-in copy.
+
+These files were converted from the original Amiga disks with
+`skids extract --data <folder with the ADFs>`. That conversion is lossless,
+and running it again overwrites your edits.
+
+## Tracks: `tracks/<name>/`
+
+### `track.png`
+
+The track picture: 1024 × 512, a 16-colour indexed (palette) image.
+
+- **Palette:** the PNG's palette is the track's palette. Change a palette
+  entry to recolour every pixel that uses it.
+- **Road vs scenery:** colours 8–15 are road (full grip, full speed).
+  Colours 0–7 are scenery (less grip, slower).
+- **Painting a new route:** paint it with the upper eight colours and
+  update the gates to match.
+- **Saving:** keep the image indexed if your editor allows it. If it saves
+  as full-colour RGB instead, each pixel is matched to the nearest default
+  colour, which loses your palette.
+
+### `track.txt`
+
+Plain text; lines starting with `#` are comments.
+
+- **`gate x1 y1 x2 y2`:** lap gates in race order.
+  - Gate 0 is the start/finish line, and the grid forms up behind it.
+  - Cars must cross every gate, in order, to complete a lap. Computer
+    drivers steer from one gate to the next.
+  - Coordinates are on the isometric ground plane. To find where a point
+    sits in the picture:
+    `picture x = wx - wy + 512`, `picture y = (wx + wy) × 31/64 − 252`.
+  - `skids trace --data assets --track N --out t.png` draws the gates
+    (gate 0 in yellow) and the computer cars' paths over the picture.
+- **`height`:** 64 rows of 64 numbers giving ground height × 256. Each
+  number covers a 16 × 8 pixel cell of the picture. Slopes push cars
+  downhill and tilt the car sprites.
+- **`start`, `flags`:** values from the original data that the game
+  doesn't use.
+
+To add a track, make a new folder with both files. Tracks are listed in
+name order.
+
+## Vehicles: `vehicles/<id>.png` and `<id>.txt`
+
+### `<id>.png`
+
+A 2048 × 800 sprite sheet: 32 headings across by 25 tilts down, each cell
+64 × 32.
+
+- **Colours:** use only palette indices 0–3:
+  - 0 transparent
+  - 1 tyres / shadow
+  - 2 trim
+  - 3 bodywork
+  
+  The colours in the file are only a preview; each player's car is
+  recoloured at run time.
+- **Headings:** column 12 faces right, and columns advance clockwise in
+  11.25° steps.
+- **Tilts:** row = roll × 5 + pitch (each 0–4), so row 12 is the car on
+  flat ground.
+
+### `<id>.txt`
+
+The display name and handling:
+
+| Setting   | Meaning |
+|-----------|---------|
+| `top`     | top speed on tarmac |
+| `accel`   | acceleration |
+| `brake`   | braking |
+| `grip`    | sideways grip; higher means less sliding |
+| `turn`    | steering rate |
+| `mass`    | weight in collisions |
+| `offroad` | speed and grip multiplier on scenery |
+
+To add a car, add a new `<id>.png` / `<id>.txt` pair.
+
+## Palettes: `palettes/*.pal` (optional)
+
+16 lines of `RRGGBB`. Each file overrides the PNG palettes:
+
+- `track1-3.pal`: one track
+- `track1.pal`: every TRACK1 track
+- `default.pal`: all tracks