Deprecate cupy array backend

[priyam0k]

Summary of Changes

Emit a DeprecationWarning whenever the cupy array backend is selected, without removing any functionality (per @henrydingliu's guidance to warn only before 1.0). Covers all three entry points a user can enable cupy through:

• Options.set_option('ARRAY_BACKEND', 'cupy') → warns.
• Options.set_option('ARRAY_PRIORITY', [...]) → warns when 'cupy' is in the list.
• Triangle.set_backend('cupy') → warns (fires once, in the inplace branch).

Added tests asserting the warnings fire and that non-cupy backends do not warn.

Related GitHub Issue(s)

Closes #843

Additional Context for Reviewers

• No behaviour is removed — cupy still works; this is the deprecation-notice step only.

• Warning messages link back to Deprecate Cupy #843 so users know where to follow up.

• Files touched: chainladder/__init__.py, chainladder/core/common.py, chainladder/utils/tests/test_utilities.py.

• 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
Notify-only deprecation with no removal of cupy behavior; limited to options and backend conversion entry points plus tests.

Overview
Adds DeprecationWarning when users select the cupy array backend, without removing cupy support (issue #843).

Warnings fire at Options.set_option for ARRAY_BACKEND='cupy' or when 'cupy' appears in ARRAY_PRIORITY, and at Triangle.set_backend('cupy') with stacklevel=2 so the warning points at caller code. set_backend gains an internal _warn flag so deep/recursive conversions do not spam duplicate warnings.

New tests assert warnings appear for cupy paths, non-cupy options stay silent, and set_backend(..., deep=True) emits exactly one warning attributed to the test file.

^{Reviewed by Cursor Bugbot for commit 8c06433. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

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

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 69ab9b0. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.10%. Comparing base (449b5c1) to head (8c06433).

Files with missing lines	Patch %	Lines
chainladder/core/common.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #881      +/-   ##
==========================================
+ Coverage   87.04%   87.10%   +0.06%     
==========================================
  Files          86       86              
  Lines        4986     4994       +8     
  Branches      646      649       +3     
==========================================
+ Hits         4340     4350      +10     
+ Misses        456      454       -2     
  Partials      190      190              

Flag	Coverage Δ
unittests	87.10% <90.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[henrydingliu]

@genedan I'd like to defer this review to you.