Allow exceptions when checking alignment

Suggestion from the COB Workshop: Some ontologies might need “exceptions” when they check their alignment with COB.

That is, they might want to explicitly tolerate that a handful of explicitly specified terms are unaligned, _without_ causing the check to fail.

Could be achieved using the existing `--term` option, but maybe a dedicated option for this case would be warranted?

Start by tracing how the existing `--term` option is handled and where the COB alignment check decides that unaligned terms cause failure. Resolve whether exceptions should reuse `--term` or use a dedicated option. Done means explicitly listed terms can remain unaligned without failing the check, with the behavior covered by tests.

Written by the indexing model from the issue text.