Miscellaneous improvements for 2025-W27

[khaeru]

• Implement classes related to <reg:SubmitStructureResponse>
  • Write to SDMX-ML.
  • Document implementation.
• Add sdmx.compare to collect logic related to comparing artefacts.
  • Remove all class-specific compare logic from .message and .model.
  • Expand tests of comparisons.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

❌ Patch coverage is 99.39940% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.10%. Comparing base (f140e40) to head (78dffe8).
⚠️ Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
sdmx/util/__init__.py	90.00%	1 Missing ⚠️
sdmx/writer/xml.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
- Coverage   98.92%   98.10%   -0.83%     
==========================================
  Files         103      105       +2     
  Lines        8686     8910     +224     
==========================================
+ Hits         8593     8741     +148     
- Misses         93      169      +76     

Files with missing lines	Coverage Δ
sdmx/compare.py	100.00% <100.00%> (ø)
sdmx/dictlike.py	97.67% <100.00%> (+1.92%)	⬆️
sdmx/format/xml/common.py	100.00% <ø> (ø)
sdmx/message.py	100.00% <100.00%> (ø)
sdmx/model/common.py	99.62% <100.00%> (-0.29%)	⬇️
sdmx/model/v21.py	100.00% <ø> (ø)
sdmx/reader/xml/common.py	100.00% <100.00%> (ø)
sdmx/testing/__init__.py	99.28% <100.00%> (+0.05%)	⬆️
sdmx/tests/model/test_common.py	100.00% <100.00%> (ø)
sdmx/tests/model/test_v21.py	100.00% <100.00%> (ø)
... and 9 more

... and 20 files with indirect coverage changes

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