blueMSX picks a cartridge's mapper by looking the ROM up in its
Databases/ XMLs; without them any megaROM cart loads, reports success,
and sits on a blank screen. But the XMLs are upstream game-identification
data -- they do not belong in this repo. build-bluemsx.sh now stages them
from the FETCHED upstream checkout alongside the C-BIOS Machines tree,
the path is gitignored, and the romdev-core-bluemsx tarball publishes
them from disk: exactly the fetched-not-vendored treatment the core wasm
binaries already get.