BUG: Kharn the Betrayer's ability doesn't play well with menace

An opponent attacks me with a creature with Menace. Kharn is my only creature.
I am forced to block due to Kharn's ability.
I can't block, due to Menace. The game gets stuck.

Reproduce the attack with Kharn the Betrayer as the only blocker against a creature with Menace. Start by tracing how Kharn's forced-block ability interacts with Menace's blocking restrictions, then identify the game state where the interaction gets stuck. Done means the game resolves this scenario without becoming stuck, with a regression test if the relevant test area is found.

Written by the indexing model from the issue text.