[BUG] dev_to_val and val_to_dev does not respect inplace

[henrydingliu]

Summary of Changes

• changed _auto_sparse to return None
• changed _val_dev to mutate inplace only
• _val_dev, grain, dev_to_val, val_to_dev now preserve backend
• dev_to_val, val_to_dev now respect inplace

Related GitHub Issue(s)

closes #1477
closes #1482

AI/LLM Usage

None

Additional Context for Reviewers

• _auto_sparse is a private method that already mutates inplace. returning the object is redundant
• cleaning up _auto_sparse was at one point relevant to the core issue of the PR. I've since removed _auto_sparse from _val_dev to address a long-standing bug where dev_to_val inadvertently changes a sparse triangle back to numpy
• _val_dev is a private helper method used in dev_to_val and val_to_dev. with the refactor of these two public methods, there is no longer a need for _val_dev to have inplace=False.

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.
• AI/LLM usage is disclosed and appropriate.
• Documentation and tests are appropriate.
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---

Note

Medium Risk
Changes behavior of widely used development/valuation conversion methods and inplace return values; incorrect backend restoration could affect sparse vs numpy performance or memory for large triangles.

Overview
Fixes dev_to_val and val_to_dev so inplace=True mutates the triangle and returns None, while inplace=False returns a new Triangle (copy-then-mutate). Docstrings now document Triangle | None return types.

The private _val_dev helper is in-place only: it temporarily switches to sparse for the coordinate reshape, then set_backend restores the caller’s original backend instead of routing through _auto_sparse (which could flip sparse triangles back to numpy during conversion).

_auto_sparse is aligned with its existing mutation behavior by returning None; aggregation paths in pandas.py and triangle construction call it without reassigning self.

Tests are updated for inplace semantics, _auto_sparse expectations, grain assertions via to_frame(), and a new test_dev_val_inplace.

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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): 15.7% of exported symbols fully typed (229 / 1457)

	Known	Ambiguous	Unknown	Total
Project (head)	229	108	1120	1457

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: 324
• Functions without default param: 0
• Classes without docstring: 8

Patch (exported symbols added or changed by this PR): 66.7% fully typed (2 / 3)

	Known	Ambiguous	Unknown	Total
Patch	2	0	1	3

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_dev_val_inplace	❌ unknown	new
chainladder.core.triangle.Triangle.dev_to_val	✅ known	changed (was ❌ unknown)
chainladder.core.triangle.Triangle.val_to_dev	✅ known	changed (was ❌ unknown)

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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.18%. Comparing base (856f87b) to head (3279003).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1480      +/-   ##
==========================================
+ Coverage   94.15%   94.18%   +0.03%     
==========================================
  Files          96       96              
  Lines        5852     5851       -1     
  Branches      739      734       -5     
==========================================
+ Hits         5510     5511       +1     
+ Misses        221      220       -1     
+ Partials      121      120       -1     

Flag	Coverage Δ
unittests	94.18% <100.00%> (+0.03%)	⬆️

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