initial implementation of TriangleSelector

[henrydingliu]

Summary of Changes

New workflow step for combining incurred/paid into a single VotingChainladder estimator

Related GitHub Issue(s)

helps with various chapters of #588

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)

---

Note

Medium Risk
Changes how VotingChainladder combines weighted ultimates, which affects ensemble reserving results whenever column dimensions differ across estimators.

Overview
Adds TriangleSelector, a sklearn-style pipeline step that narrows a Triangle to a single column (e.g. paid vs incurred) so each branch of a VotingChainladder can run on different measures from the same input.

VotingChainladder blending in _get_ultimate is updated so weighted combinations use each estimator’s ultimate_ shape instead of assuming it matches the full input triangle; mismatched shapes raise a ValueError with a hint about TriangleSelector. The class is exported from chainladder and covered by workflow tests plus the public API guard.

^{Reviewed by Cursor Bugbot for commit dfba575. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 14.6% of exported symbols fully typed (187 / 1280)

	Known	Ambiguous	Unknown	Total
Project (head)	187	106	987	1280

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 315
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 25.0% fully typed (2 / 8)

	Known	Ambiguous	Unknown	Total
Patch	2	0	6	8

Patch symbol details

Symbol	Status	Change
chainladder.workflow.TriangleSelector	❌ unknown	new
chainladder.workflow.tests.test_voting.test_mismatching_tri_sel	❌ unknown	new
chainladder.workflow.tests.test_voting.test_tri_sel	❌ unknown	new
chainladder.workflow.voting.TriangleSelector	❌ unknown	new
chainladder.workflow.voting.TriangleSelector.__init__	✅ known	new
chainladder.workflow.voting.TriangleSelector.col	✅ known	new
chainladder.workflow.voting.TriangleSelector.fit	❌ unknown	new
chainladder.workflow.voting.TriangleSelector.transform	❌ unknown	new

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.55%. Comparing base (32ae098) to head (dfba575).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1114      +/-   ##
==========================================
+ Coverage   90.50%   90.55%   +0.04%     
==========================================
  Files          91       91              
  Lines        5289     5306      +17     
  Branches      671      672       +1     
==========================================
+ Hits         4787     4805      +18     
  Misses        359      359              
+ Partials      143      142       -1     

Flag	Coverage Δ
unittests	90.55% <100.00%> (+0.04%)	⬆️

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