[Code scan] Julia Pardiso backend crashes for orthogonal exports without overlap file

[njzjz]

This issue is part of a Codex global repository scan.

Problem:
to_pardiso_json() writes predicted_overlaps.h5 only when sr is not None, which is expected for orthogonal models. The Julia loader returns an empty overlap dict when the file is absent, but HR2HK() unconditionally indexes S_R[R] for every Hamiltonian lattice shift.

Code references:
https://github.com/deepmodeling/deeptb/blob/86c60c73996f0dd961c3138f2e88424382cb734e/dptb/postprocess/unified/system.py#L832-L839
https://github.com/deepmodeling/deeptb/blob/86c60c73996f0dd961c3138f2e88424382cb734e/dptb/postprocess/pardiso/io/io.jl#L172-L176
https://github.com/deepmodeling/deeptb/blob/86c60c73996f0dd961c3138f2e88424382cb734e/dptb/postprocess/pardiso/utils/hamiltonian.jl#L66-L70

Impact:
Orthogonal Pardiso runs can fail with a missing-key error instead of using an identity overlap matrix.

Suggested fix:
Teach the Julia backend to synthesize identity overlap blocks when no overlap file is present, or have the Python exporter always write an explicit identity overlap for orthogonal models.