build.rs unpacks the ADF images next to Cargo.toml (or in
SKIDS_EMBED_DIR) with the same ADF/LHA readers the game uses and packs
the files into the executable. At run time the built-in data loads
first and anything given with --data, or found next to the executable,
overrides it. The embed feature is on by default; builds without it
behave as before and ask for --data. CI now runs the binary from an
empty directory.
