Placeholder so `tests/perf/lubm/data/` exists in fresh clones.

The actual generated LUBM dataset lives in the docker named volume
`pgrdf-lubm-data` (created by `just lubm-build` / `just lubm-gen N`),
not on the host filesystem. This file keeps the directory present so
local scripts can bind-mount through it if a teammate prefers a host
path over a named volume.

See `../README.md` for the full TF-12 intake.
