document lack of end layers fitting #373

[rozyczko]

The gradient discretization now always includes both endpoints (front and back materials), using linspace for even spacing and eliminating off-by-one errors.
The minimum allowed number of discretization elements is now enforced as 2 (front and back), with improved error messages

The GradientLayer class and its documentation now make it explicit that the front and back materials are frozen at construction and are excluded from the parameter tree, preventing them from being offered as fittable parameters. This avoids silent no-ops if a user tries to fit them.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 42.44%. Comparing base (5e14e7f) to head (156e4b4).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...yreflectometry/sample/assemblies/gradient_layer.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #386       +/-   ##
============================================
+ Coverage     0.00%   42.44%   +42.44%     
============================================
  Files           42       48        +6     
  Lines         3573     3824      +251     
============================================
+ Hits             0     1623     +1623     
+ Misses        3573     2201     -1372     

Flag	Coverage Δ
integration	42.44% <88.88%> (+42.44%)	⬆️
unittests	0.00% <0.00%> (ø)

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Files with missing lines	Coverage Δ
...yreflectometry/sample/assemblies/gradient_layer.py	85.71% <88.88%> (+85.71%)	⬆️

... and 43 files with indirect coverage changes

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