[FEAT] IncrementalAdditive Cleanup for Chapter 16 Part 1

[henrydingliu]

Summary of Changes

More clean-up and bug fix as I work through Chapter 16

• Cleaning up zeta_-related properties into IncrementalMixin, matching DisposalMixin
• fixed a bug in cum_to_incr for cum_zeta_, w/ new test to guard future drift; also improved readability of the surrounding code
• refactored _param_property across modules to condense the universal properties to DevelopmentBase and keep module-specific properties at module level
• other docstring improvements

Related GitHub Issue(s)

#588
#377

AI/LLM Usage

AI did some autofill of docstrings, which I then validated edited

Additional Context for Reviewers

zeta_ still shows the wrong development labels. will address after #1304

Submitter's Checklist

• I have reviewed and am adhering to the standards outlined in the project Governing Doc.
• The PR subject title summarizes the changes, with one proper prefix ([FIX], [FEAT], [DOCS], [TST], [CHORE], or [BRK]).
• I am a human (not a bot), and this PR form is written by a human.

Reviewer's Checklist

• The implementation addresses the associated issue(s).
• The implementation is appropriate, maintainable, and follows ARCHITECTURE.md.
• PR subject title has the proper prefix and the subject is appropriate.
• Relevant issue(s) are linked.
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---

Note

Medium Risk
Touches core triangle cum/incr logic and how fitted parameters are stored and exposed; behavior changes for additive patterns are guarded by new tests but affect development workflows.

Overview
Introduces IncrementalMixin (mirroring DisposalMixin) so zeta_ / cum_zeta_ live on triangles and estimators with backing _zeta_, enforced pattern flags on set, and has_zeta keyed off _zeta_. cum_zeta_ is removed from common in favor of the mixin.

incr_to_cum / cum_to_incr on Triangle are refactored to branch explicitly on is_pattern, is_additive, and is_disposal_rate, fixing incorrect incremental conversion for additive zeta_ / cum_zeta_ (new is_additive property). DevelopmentBase._param_property only applies shared wrapping; Development, IncrementalAdditive, and disposal each override for module-specific values and flags.

Tests cover cum_zeta_ assignment and missing-zeta_ errors.

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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 / 1387)

	Known	Ambiguous	Unknown	Total
Project (head)	209	111	1067	1387

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 (2 / 6); 1 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	2	0	4	6

Patch symbol details

Symbol	Status	Change
chainladder.core.triangle.Triangle.is_additive	✅ known	new
chainladder.development.incremental.IncrementalMixin	❌ unknown	new
chainladder.development.incremental.IncrementalMixin.cum_zeta_	❌ unknown	new
chainladder.development.incremental.IncrementalMixin.zeta_	❌ unknown	new
chainladder.development.tests.test_incremental.test_no_zeta	❌ unknown	new
chainladder.development.tests.test_incremental.test_setting_cum	✅ known	new
chainladder.core.common.Common.cum_zeta_	—	no longer exported (was ❌ unknown)

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Codecov Report

❌ Patch coverage is 96.36364% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.74%. Comparing base (d82c521) to head (d461b14).
⚠️ Report is 108 commits behind head on main.

Files with missing lines	Patch %	Lines
chainladder/core/triangle.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1419      +/-   ##
==========================================
+ Coverage   91.79%   92.74%   +0.95%     
==========================================
  Files          96      102       +6     
  Lines        5494     5967     +473     
  Branches      707      742      +35     
==========================================
+ Hits         5043     5534     +491     
+ Misses        327      312      -15     
+ Partials      124      121       -3     

Flag	Coverage Δ
unittests	92.74% <96.36%> (+0.95%)	⬆️

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[henrydingliu]

@genedan just assigned this to you as i'd like this to land so i can continue working on chapter 16.