[FIX] Apply Ruff fixes to core files.

[genedan]

Summary of Changes

Mostly mundane formatting with the exception of one variable name change (see #1285). I left display untouched as that will be addressed in #1284 and an rtest file that needs further debugging.

Related GitHub Issue(s)

#1216

Additional Context for Reviewers

Checklist

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

---

Note

Low Risk
Mostly formatting and clearer naming in correlation helpers; no intended algorithm or slicing behavior changes.

Overview
Applies Ruff lint/format cleanup across chainladder/core/correlation.py, slice.py, and related core tests, and drops the corresponding per-file ignores in pyproject.toml now that those files pass the rules.

The only non-cosmetic code changes are in Mack correlation code: the development-period count variable I → n_dev_periods (with a Mack-97 comment), and the inner helper pZlower → p_z_lower. Formulas and call sites are updated to use the new names; behavior should be unchanged.

Tests are adjusted for style (e.g. raa.__eq__(None) instead of == None for Ruff) and test_grain.py drops unused imports.

^{Reviewed by Cursor Bugbot for commit 025afa4. 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): 15.4% of exported symbols fully typed (213 / 1381)

	Known	Ambiguous	Unknown	Total
Project (head)	213	111	1057	1381

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: 325
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): no exported symbol type-completeness changes detected.

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.68%. Comparing base (670e86a) to head (025afa4).

Files with missing lines	Patch %	Lines
chainladder/core/slice.py	97.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1287      +/-   ##
==========================================
- Coverage   91.68%   91.68%   -0.01%     
==========================================
  Files          93       93              
  Lines        5447     5445       -2     
  Branches      704      704              
==========================================
- Hits         4994     4992       -2     
  Misses        328      328              
  Partials      125      125              

Flag	Coverage Δ
unittests	91.68% <97.82%> (-0.01%)	⬇️

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