feat(librpa): export SOC magnetic symmetry metadata

[AroundPeking]

Summary

• Export spatial and magnetic symmetry operations in stru_out.txt for LibRPA.
• Preserve a common operation-table layout for ordinary space groups and magnetic groups.
• For nspin = 4, export spin_symmetry <grey_group> 2 flags so LibRPA can reconstruct SOC spin rotations from spatial operations.
• Add a compact SOC-plus-RPA producer regression and serializer unit coverage.
• Document the SOC magnetic-symmetry export format.

[github-actions]

Agent Governance Check

Severity	Rule	Location	Reason	Suggested action	Exception
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:4	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:5	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:6	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:8	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:9	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:10	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:11	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/librpa_stru_symmetry.h:12	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	Header dependency review	source/source_lcao/module_ri/rpa_lri.hpp:27	Header diff adds an include dependency.	Confirm the declaration requires this include; prefer forward declarations where practical.	allowed
warning	PR metadata completeness	pull_request.body	missing sections: Linked Issue, Unit Tests and/or Case Tests for my changes, What's changed?	Fill the PR template with issue linkage, test evidence, and a concise change summary.	allowed