feat: reuse relaxation CHGCAR for init MD

[njzjz-bot]

Summary

• add the optional VASP init parameter reuse_relax_chgcar
• download CHGCAR from each stage-1 relaxation and stage it for matching stage-3 MD tasks
• fail clearly if reuse is enabled but a system has no relaxation CHGCAR
• test schema validation, local linking, and dispatcher transfer lists

Users remain responsible for compatible VASP restart settings in the MD INCAR.

Testing

• /tmp/dpgen-test-env/bin/python -m unittest tests.data.test_init_chgcar tests.data.test_gen_bulk tests.test_check_examples -v
• ruff check dpgen/data/gen.py dpgen/data/arginfo.py tests/data/test_init_chgcar.py
• ruff format --check dpgen/data/gen.py dpgen/data/arginfo.py tests/data/test_init_chgcar.py

Fixes #1836

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

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📥 Commits

Reviewing files that changed from the base of the PR and between d5ce577 and a07853e.

📒 Files selected for processing (3)

• dpgen/data/arginfo.py
• dpgen/data/gen.py
• tests/data/test_init_chgcar.py

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[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.46%. Comparing base (d5ce577) to head (a07853e).

Files with missing lines	Patch %	Lines
dpgen/data/gen.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1963      +/-   ##
==========================================
+ Coverage   49.80%   50.46%   +0.66%     
==========================================
  Files          83       83              
  Lines       14986    14998      +12     
==========================================
+ Hits         7464     7569     +105     
+ Misses       7522     7429      -93     

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