Use latest release tag as ABI check baseline

[thetic]

Description

The abi_check job was using ${{ github.base_ref }} (HEAD of the target branch) as the baseline for abidiff. This means the check compared against unreleased in-flight changes rather than what users have actually shipped. Switching to the latest GitHub release tag gives the check meaningful semantics: "does this PR break the ABI of the last published release?"

Related Issues

Fixes # (issue number)

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Manual Verification (Optional)

N/A — CI-only change.

Checklist

• I have written/updated documentation in docs/ for any user-facing changes.
• My code follows the project's naming conventions (mu::tiny namespace, INCLUDED_MUTINY_ guards, mutiny_ C-prefix).
• For new features, I have considered if a C-interface adapter (.h and .c.cpp) is required for parity.
• I have reviewed the CONTRIBUTING.md file to ensure compliance with architectural guidelines.

[coveralls]

coverage: 98.867%. remained the same — base-camp into main