Sparse fixes

[genedan]

Summary of Changes

Remove blockage for #1157.

Related GitHub Issue(s)

#1155

Additional Context for Reviewers

Trying to fix this revealed some bugs, mainly in-place assignment to Triangle.values with a sparse backend, which I explicitly forbid (earlier, would have just failed with a cryptic error message). The other was assignment to a Triangle column, which had a bug.

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

---

Note

Medium Risk
Changes core Triangle value storage and column assignment on the sparse path, which many estimators rely on; risk is mitigated by expanded tests and clearer failure modes for unsupported in-place mutation.

Overview
Fixes sparse backend behavior around column assignment, numpy interoperability, and mutating values, unblocking related work on sparse triangles (#1155 / #1157).

Sparse column assignment (Triangle[...] = value): Replacing an existing column no longer rebuilds via drop(key); it filters the target column out of the current COO coordinates and merges new data. Assignments can use a Triangle or a raw COO array, aligned with the numpy path. Misaligned new-column concatenation also catches AssertionError.

In-place values writes on sparse are explicitly rejected: COO.__setitem__ raises a clear TypeError directing users to the numpy backend (instead of obscure failures). __array__ on sparse triangles returns a dense array so np.asarray(triangle) behaves predictably.

CI adds workflow concurrency so duplicate pytest runs on the same branch/ref cancel in-progress jobs.

Tests are updated to run on both backends where relevant and to expect or skip in-place values mutation on sparse.

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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): 14.9% of exported symbols fully typed (194 / 1300)

	Known	Ambiguous	Unknown	Total
Project (head)	194	110	996	1300

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: 314
• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (a9f45e1) to head (254ca05).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1164      +/-   ##
==========================================
+ Coverage   90.94%   91.27%   +0.32%     
==========================================
  Files          91       91              
  Lines        5325     5422      +97     
  Branches      675      691      +16     
==========================================
+ Hits         4843     4949     +106     
+ Misses        344      339       -5     
+ Partials      138      134       -4     

Flag	Coverage Δ
unittests	91.25% <100.00%> (+0.30%)	⬆️

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[henrydingliu]

passes sparse tests here