Separate category-owning sections from real CIF datablocks

[AndrewSazonov]

This change reorganizes how the library models the different pieces of a
project that own CIF-like categories. Crystal structures and experiments
stay "real" CIF data blocks — each one is saved with its own data_<id>
header, exactly as before. The Analysis section (fitting mode,
aliases, constraints, joint/sequential fit settings) and project-level
configuration are now treated as singleton sections that share the same
convenient features (category discovery, parameter listing, dirty
tracking, help() tables) without pretending to be data blocks. As a
result:

[codecov]

Codecov Report

❌ Patch coverage is 90.44118% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.94%. Comparing base (6a4887e) to head (df93079).

Files with missing lines	Patch %	Lines
...easydiffraction/project/categories/info/default.py	84.44%	12 Missing and 2 partials ⚠️
src/easydiffraction/io/cif/serialize.py	75.75%	3 Missing and 5 partials ⚠️
src/easydiffraction/core/variable.py	83.33%	0 Missing and 2 partials ⚠️
src/easydiffraction/core/category_owner.py	97.50%	0 Missing and 1 partial ⚠️
src/easydiffraction/project/project.py	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #176      +/-   ##
===========================================
- Coverage    86.01%   85.94%   -0.08%     
===========================================
  Files          168      173       +5     
  Lines        13231    13335     +104     
  Branches      1557     1564       +7     
===========================================
+ Hits         11381    11461      +80     
- Misses        1275     1295      +20     
- Partials       575      579       +4     

Flag	Coverage Δ
integration	69.96% <84.55%> (-0.05%)	⬇️
unittests	78.38% <90.07%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easydiffraction/analysis/analysis.py	86.97% <100.00%> (+0.85%)	⬆️
src/easydiffraction/core/datablock.py	81.81% <100.00%> (-5.96%)	⬇️
...asydiffraction/project/categories/info/__init__.py	100.00% <100.00%> (ø)
...easydiffraction/project/categories/info/factory.py	100.00% <100.00%> (ø)
...iffraction/project/categories/rendering/default.py	88.88% <100.00%> (+0.82%)	⬆️
src/easydiffraction/project/project_config.py	100.00% <100.00%> (ø)
src/easydiffraction/project/project_info.py	100.00% <100.00%> (+11.66%)	⬆️
src/easydiffraction/core/category_owner.py	97.50% <97.50%> (ø)
src/easydiffraction/project/project.py	87.30% <93.33%> (-0.05%)	⬇️
src/easydiffraction/core/variable.py	90.53% <83.33%> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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