- pin lmdb submodule to openldap LMDB_1.0.2 (c279e047); link module.c
  so the build matches upstream liblmdb.a (mdb.o midl.o module.o)
- PackageDescription 6.0; drop the empty shim lists from the manifest
- add a Swift Testing smoke suite (version, compile-time macros,
  write/read cycle through the real engine)
- document engine pin, submodule workflow, and the 0.9->1.0 data
  format break in README