The gen_server API port itself already landed upstream (078072b, tracking
smarm HEAD efbc254 pre-git-dep). This just moves the dependency off the
local path checkout onto the git remote, pinned to the smarm v0.5.0
release tag (a03a7ca) rather than a floating branch HEAD.
Verified: 90 lib + 45 integration + 2 doc tests green, examples build,
under default and --all-features.