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.

Cargo.toml

@@ -12,8 +12,8 @@ default = ["audio", "embed"]
 # Procedural engine/skid sound via cpal. Disable with --no-default-features
 # on Linux machines without the ALSA development headers.
 audio = ["dep:cpal"]
-# Build the game data from the ADF images next to Cargo.toml (or in
-# SKIDS_EMBED_DIR) into the executable, so it runs without the disks.
+# Build the editable game assets in assets/ (or SKIDS_ASSETS_DIR) into the
+# executable, so it runs on its own.
 embed = []
 
 [dependencies]