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.

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
         run: cargo test --release
       - name: Build
         run: cargo build --release
-      - name: Check the binary runs without the disks
+      - name: Check the binary runs on its own
         shell: bash
         run: |
           mkdir -p "$RUNNER_TEMP/standalone"