Add Slicing tests, annotations.

[genedan]

Summary of Changes

Adding some tests for uncovered lines in slice.py. Add some docstrings and typing for Locaction class.

Related GitHub Issue(s)

#486

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

Low Risk
Changes are mostly annotations, docs, and tests; sparse COO rebuild is a small refactor in an already-used code path with new tests guarding behavior.

Overview
Adds typing, module docstring, and docstrings across Triangle .loc/.iloc slicing in slice.py, including a new _LabelKey alias in typing.py for label-based indexers.

Implementation tweaks: sparse._slicing is loaded via importlib instead of a direct import; sparse single-cell updates in _sparse_setitem use explicit COO typing and keyword construction; several type() checks become isinstance.

Tests: test_slicing.py gains stricter assertions (origin/development bounds, missing assert on link_ratio), coverage for invalid .loc column keys, .at assignment from a TriangleSlicer, and minor hygiene (_= for intentional reads).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (11f6263) to head (219b586).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #946      +/-   ##
==========================================
+ Coverage   88.69%   88.79%   +0.09%     
==========================================
  Files          89       89              
  Lines        5052     5060       +8     
  Branches      645      646       +1     
==========================================
+ Hits         4481     4493      +12     
+ Misses        425      423       -2     
+ Partials      146      144       -2     

Flag	Coverage Δ
unittests	88.79% <100.00%> (+0.09%)	⬆️

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[henrydingliu]

is this doing anything? do you want to make it into another assert?

[genedan]

I went ahead and added some assertions in lines that didn't have any. There are still a couple outstanding regarding .iat and .at, I have yet to annotate/explore those classes in slicing.py so I've left those for a future PR - probably 2 steps into the future as TriangleSlicer is next on the PR queue. The one after that will likely combine work onAT/IAT/VirtualColumns to complete the slicing annotations & coverage gaps.