Improve parameter discoverability, constraint semantics, and CIF output

[AndrewSazonov]

This PR improves API discoverability and parameter inspection across the project by adding help() coverage for display and summary facades, hiding large loop-backed categories from parameter summary views, and adding a Quick Reference page for common workflows.

It also clarifies fitting semantics by renaming constraint state flags to distinguish user-defined constraints from symmetry-imposed constraints, aligning fittable displays with actual refinability, and fixing CIF/identity issues so loop-backed items resolve the correct datablock path and empty category fragments do not introduce blank sections in show_as_cif() output.

[codecov]

Codecov Report

❌ Patch coverage is 91.82390% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.32%. Comparing base (efadbcd) to head (c566d6e).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/easydiffraction/utils/utils.py	86.53%	2 Missing and 5 partials ⚠️
src/easydiffraction/analysis/analysis.py	90.24%	0 Missing and 4 partials ⚠️
...ablocks/structure/categories/atom_sites/default.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #172      +/-   ##
===========================================
+ Coverage    86.29%   86.32%   +0.02%     
===========================================
  Files          158      158              
  Lines        12138    12179      +41     
  Branches      1394     1400       +6     
===========================================
+ Hits         10475    10513      +38     
- Misses        1154     1155       +1     
- Partials       509      511       +2     

Flag	Coverage Δ
integration	70.69% <85.53%> (+<0.01%)	⬆️
unittests	78.07% <87.42%> (+0.27%)	⬆️

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

Files with missing lines	Coverage Δ
src/easydiffraction/analysis/fitting.py	96.49% <ø> (ø)
src/easydiffraction/analysis/sequential.py	71.23% <100.00%> (ø)
src/easydiffraction/core/collection.py	96.38% <100.00%> (+0.33%)	⬆️
src/easydiffraction/core/datablock.py	87.77% <100.00%> (ø)
src/easydiffraction/core/singleton.py	82.00% <100.00%> (ø)
src/easydiffraction/core/variable.py	90.94% <100.00%> (ø)
...easydiffraction/crystallography/crystallography.py	89.21% <100.00%> (ø)
.../datablocks/experiment/categories/data/bragg_pd.py	98.21% <100.00%> (ø)
.../datablocks/experiment/categories/data/total_pd.py	97.60% <100.00%> (ø)
...datablocks/experiment/categories/refln/bragg_sc.py	97.97% <100.00%> (ø)
... and 8 more

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