fix(lammps): emit valid spin atom style sections

[njzjz-bot]

Fixes #885.

Mark spin-vector data as Atoms # spin, support automatic spin-style parsing, and validate the generated file with LAMMPS.

Tests: cd tests && python -m unittest test_lammps_spin (LAMMPS integration skips when the executable/package is unavailable).

Why existing tests missed it: The previous test only inspected text; it emitted nine-column spin rows under an atomic-style header that LAMMPS rejects.

Coding agent: Codex
Codex version: codex-cli 0.144.4
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 0416b54 and c7cdaac.

📒 Files selected for processing (3)

• dpdata/formats/lammps/lmp.py
• dpdata/plugins/lammps.py
• tests/test_lammps_spin.py

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[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 2 untouched benchmarks

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_{Comparing njzjz-bot:fix/issue-885 (c7cdaac) with master (0416b54)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.87%. Comparing base (0416b54) to head (c7cdaac).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1032   +/-   ##
=======================================
  Coverage   86.87%   86.87%           
=======================================
  Files          89       89           
  Lines        8266     8269    +3     
=======================================
+ Hits         7181     7184    +3     
  Misses       1085     1085           

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