promoting _param_property

[henrydingliu]

Summary of Changes

Related GitHub Issue(s)

Additional Context for Reviewers

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run jb build docs --builder=custom --custom-builder=doctest)

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Note

Low Risk
Small refactor with no API change; Development still uses its own three-argument _param_property for LDF/sigma/std_err.

Overview
Moves _param_property from IncrementalAdditive onto DevelopmentBase as a shared static helper that wraps fitted parameter arrays into a pattern Triangle (ultimate valuation, additive/non-cumulative flags, slicers).

IncrementalAdditive.fit now calls the inherited helper instead of a local duplicate. _set_fit_groups type hints switch from TriangleLike to concrete Triangle under TYPE_CHECKING.

^{Reviewed by Cursor Bugbot for commit d189394. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.17%. Comparing base (0f59a1b) to head (d189394).
⚠️ Report is 16 commits behind head on promote_param_property.

Additional details and impacted files

@@                    Coverage Diff                     @@
##           promote_param_property    #1017      +/-   ##
==========================================================
+ Coverage                   88.94%   89.17%   +0.23%     
==========================================================
  Files                          89       89              
  Lines                        5139     5166      +27     
  Branches                      660      661       +1     
==========================================================
+ Hits                         4571     4607      +36     
+ Misses                        398      394       -4     
+ Partials                      170      165       -5     

Flag	Coverage Δ
unittests	89.17% <100.00%> (+0.23%)	⬆️

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