May 2026 release v3

Use bundled CBC binary instead of system cbc in PuLP solver (#116)

* Fix invalid GitHub Actions versions (checkout@v6, setup-python@v6 don't exist)

Use actions/checkout@v4 and actions/setup-python@v5.

https://claude.ai/code/session_01WB3kJ5V9gTucNBFeAPcfbT

* Fix CBC crashes caused by cbcbox unstable build in solution enumeration

The cbcbox package (installed via pulp[cbc]) registers an unstable/dev
CBC binary as the system `cbc` command. This build crashes in symmetry
detection (CbcNauty::computeAuto) when re-solving a problem with added
blocking constraints, silently truncating the enumeration loop after
~6 solutions.

Pin get_solver('cbc') to PuLP's own stable bundled CBC 2.10.3 binary
via pulp_cbc_path to bypass the cbcbox binary.

Also revert the erroneous actions/checkout@v4 and actions/setup-python@v5
downgrade from the previous commit — v6 of both actions exists and is correct.

https://claude.ai/code/session_01WB3kJ5V9gTucNBFeAPcfbT

* Add TODO to remove cbcbox workaround once stable CBC is available

https://claude.ai/code/session_01WB3kJ5V9gTucNBFeAPcfbT

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Co-authored-by: Claude <noreply@anthropic.com>