docs: add Triangle loc/iloc/at/iat examples (#704)

[EKtheSage]

Summary of Changes

• Add doctest Examples for Triangle.loc, .iloc, .at, .iat, column [] selection, and column assignment in chainladder/core/slice.py.
• Document loc / iloc / at / iat as properties so the examples appear on the Triangle API page (instead of being excluded as attributes).
• Keep indexers pointing at the current Triangle after copy() / slicing by constructing them on access.

Related GitHub Issue(s)

• API Reference Examples #704

Additional Context for Reviewers

This is the first of the remaining core-module PRs for #704 (slice.py is the highest-priority leftover). Follow-ups: base.py, common.py, display.py, dunders.py, io.py, and remaining pandas.py methods.

loc/iloc/at/iat are now properties that always wrap self. _set_slicers only refreshes virtual_columns. Slicing tests: 72 passed; triangle+slicing: 345 passed.

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)

Made with Cursor

[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 (201 / 1333)

	Known	Ambiguous	Unknown	Total
Project (head)	201	111	1021	1333

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: 321
• 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 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.28%. Comparing base (272cbe1) to head (d3c2059).

Files with missing lines	Patch %	Lines
chainladder/core/triangle.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1208      +/-   ##
==========================================
+ Coverage   91.25%   91.28%   +0.02%     
==========================================
  Files          91       91              
  Lines        5400     5413      +13     
  Branches      691      692       +1     
==========================================
+ Hits         4928     4941      +13     
  Misses        338      338              
  Partials      134      134              

Flag	Coverage Δ
unittests	91.28% <96.55%> (+0.02%)	⬆️

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[henrydingliu]

@genedan there's some restructuring here on slice. could you please review?

[EKtheSage]

Merged main.

Ruff: this PR touches triangle.py (dropped the duplicate loc/iloc attribute lines), so CI clears ignores on that file too. Same E721/E731/F401/F841/E712/E2 cleanup as #1212.

Doctest: loc/iloc/at/iat were in the default group and reused tri from testsetup, so other Triangle-page examples overwrote it (KeyError: paid, empty slices). Each accessor now has its own named group; tri is built in testcode.