Use declarative category identity metadata

[AndrewSazonov]

This change makes category and loop-row identities easier to audit and keeps saved CIF identifiers explicit. Constraint rows gain a stable identifier while existing constraint expressions continue to work.

[codecov]

Codecov Report

❌ Patch coverage is 97.53086% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.98%. Comparing base (aa44c46) to head (bd14001).

Files with missing lines	Patch %	Lines
...raction/analysis/categories/constraints/default.py	94.44%	0 Missing and 1 partial ⚠️
src/easydiffraction/core/category.py	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #177      +/-   ##
===========================================
+ Coverage    85.94%   85.98%   +0.03%     
===========================================
  Files          173      173              
  Lines        13335    13374      +39     
  Branches      1564     1574      +10     
===========================================
+ Hits         11461    11499      +38     
+ Misses        1295     1293       -2     
- Partials       579      582       +3     

Flag	Coverage Δ
integration	70.00% <93.82%> (+0.03%)	⬆️
unittests	78.45% <97.53%> (+0.07%)	⬆️

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

Files with missing lines	Coverage Δ
...diffraction/analysis/categories/aliases/default.py	100.00% <100.00%> (ø)
...diffraction/analysis/categories/fitting/default.py	100.00% <100.00%> (ø)
...ffraction/analysis/categories/joint_fit/default.py	100.00% <100.00%> (ø)
...tion/analysis/categories/sequential_fit/default.py	98.18% <100.00%> (ø)
...lysis/categories/sequential_fit_extract/default.py	95.94% <100.00%> (ø)
src/easydiffraction/core/identity.py	100.00% <100.00%> (ø)
...ocks/experiment/categories/background/chebyshev.py	81.94% <100.00%> (+1.38%)	⬆️
...s/experiment/categories/background/line_segment.py	93.15% <100.00%> (+1.36%)	⬆️
...locks/experiment/categories/calculation/default.py	82.75% <100.00%> (ø)
.../datablocks/experiment/categories/data/bragg_pd.py	98.21% <100.00%> (ø)
... and 19 more

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