Fix checkpoint restart file check for single-process runs

[Copilot]

The checkpoint restart sanity check hard-coded a ".1" suffix, but Run() only appends rank suffixes when numprocs>1. Single-process restarts would always fail.

Changes

• src/OutHDF5.cc (lines 196-199): Added conditional rank suffix logic matching Run():

  if (chkpt) {
    fname << outdir << "checkpoint_" << runtag;
    if (numprocs>1) fname << ".1";  // Only append rank for multi-process
  }

Now checks for checkpoint_<runtag> in single-process runs and checkpoint_<runtag>.1 in multi-process runs, consistent with how files are written in Run() (lines 309-312).

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[The9Cat]

Looks good to me.