The Gibbs ensemble simulation would deadlock when both moltransrot and
gibbs_matter moves were enabled. Root cause was RNG desynchronization
between MPI processes.

Problem:
- MoveCollection::sample() uses mpi.random to select moves (must stay in sync)
- GibbsMatterMove::_move() was using mpi.random for molecule selection
- The two cells have different molecule counts (e.g., 5 vs 15)
- std::uniform_int_distribution uses rejection sampling, which makes
  variable numbers of engine calls for different ranges
- This caused mpi.random states to diverge between processes
- Subsequent sample() calls returned different moves on each process
- When one selected gibbs_matter (with MPI calls in bias()) and the
  other selected moltransrot, the first blocked waiting for MPI

Fix:
1. Use Faunus::random (local, non-MPI) for molecule selection instead
   of mpi.random. This keeps mpi.random synchronized for move selection.

2. Add MPI exchange to synchronize early returns. When one process fails
   (no molecules found or cell full) while the other succeeds, both must
   skip the move to avoid one blocking in bias() waiting for MPI calls.