False negative missing then → than

**Describe the bug**

We have linters that correct mixing up "then" and "than" but they failed to flag the mistake in this sentence:

> If I do include cryptocurrencies abbreviations, than BTC for sure, maybe ETH, and possibly even DOGE.

Start by reproducing the quoted sentence in Harper and trace the rule or test coverage for confusing “then” and “than” cases. Add a regression test for the sentence and verify that the checker flags the incorrect “than” usage without introducing a false positive for valid comparisons.

Written by the indexing model from the issue text.