[FEAT] Fill method + Chapter 16 data

[henrydingliu]

Summary of Changes

Prep for Friedland Chapter 16

• add Triangle.fill method, with tests
• fix typos in data

Related GitHub Issue(s)

closes #1360

AI/LLM Usage

I used AI to find the incorrect data points and to debug

Additional Context for Reviewers

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Reviewer's Checklist

• The implementation addresses the associated issue(s).
• The implementation is appropriate, maintainable, and follows ARCHITECTURE.md.
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---

Note

Low Risk
Small additive API with tests and sample-data fixes; no changes to reserving or auth paths.

Overview
Adds Triangle.fill, which sets every valid (non-masked) cell to a scalar while preserving the triangle’s NaN structure via nan_triangle. Default is copy-out; inplace=True mutates values in place using the active array backend.

Tests cover a sliced triangle and a chainladder full_triangle_.

friedland_autoprop.csv is corrected for Friedland Chapter 16 prep (paid/incurred typos on two 2001 origin rows).

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[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.1% of exported symbols fully typed (209 / 1385)

	Known	Ambiguous	Unknown	Total
Project (head)	209	111	1065	1385

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: 327
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 33.3% fully typed (1 / 3)

	Known	Ambiguous	Unknown	Total
Patch	1	0	2	3

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_fill	❌ unknown	new
chainladder.core.tests.test_triangle.test_full_fill	❌ unknown	new
chainladder.core.triangle.Triangle.fill	✅ known	new

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.80%. Comparing base (01bf6bf) to head (40e843b).
⚠️ Report is 3 commits behind head on feature/fill.

Additional details and impacted files

@@               Coverage Diff                @@
##           feature/fill    #1379      +/-   ##
================================================
+ Coverage         91.78%   91.80%   +0.02%     
================================================
  Files                96       96              
  Lines              5488     5502      +14     
  Branches            707      708       +1     
================================================
+ Hits               5037     5051      +14     
  Misses              327      327              
  Partials            124      124              

Flag	Coverage Δ
unittests	91.80% <100.00%> (+0.02%)	⬆️

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[kennethshsu]

@henrydingliu can you work off a branch on casact so I can switch to it a bit easier?

Are we still doing the check if the triangle is full? I think it could be a useful hyper parameter. The current implementation checks for nans only? So while the lower half will be nan, nothing is filled, but if the upper half has a nan, it will stay as nan? Anyways I could be missing something as I am just staring at the code without being able to play with it.