Prevent bundled CBC crashes with CFFI 2.x on Linux

[zack-dev-cm]

Fresh pip installations of the declared mip==1.16rc0 can select CFFI 2.x and abort during tests/test_markets.py::test_one_interconnector on Linux x86-64. In controlled runs on unchanged master 2d3cef0, the test passes with CFFI 1.17.1 and aborts with 2.0.0 on both Python 3.9 and 3.12. CFFI 2.1.1 also aborts on Python 3.12; all other installed packages were identical in that comparison.

This constrains CFFI to >=1.17.1,<2 on the affected platform while retaining the declared solver and Python minimum. The lock already selected 1.17.1; its root metadata now matches the package version, pandas requirement, and new constraint. Other platforms retain the solver's existing CFFI requirement.

The new CI job installs a freshly built wheel in a separate environment and runs the existing offline market and solver tests outside the checkout. It includes the original interconnector test without changing or skipping it.

Validation: installed-wheel CI passed, with 56 tests and zero failures or skips on each of Python 3.9 and 3.12. uv lock --check also passed. This is a compatibility constraint for the bundled solver; it does not claim a fix to CFFI internals or validation of historical cache-dependent benchmarks.

This change is independent of #43 and can be reviewed separately.

[zack-dev-cm]

Could a maintainer approve the pending test and CBC compatibility workflows for this PR, and the test workflow for #43? The linked installed-wheel validation passes all 56 offline tests, including the original interconnector case, on Python 3.9 and 3.12. This dependency correction remains independently reviewable from #43's ramp-input changes.