Modal double-faced cards can be turned face down while on the battlefield

In Xmage, if a player controls [[Tergrid, God of Fright]] (a modal double-faced card), and an [[Ixidron]] enters the battlefield, Tergrid is incorrectly turned "face down" and becomes a 2/2 with no abilities. The rule is that double-faced cards *should not* be able to be turned face down while on the battlefield (e.g. due to the effects of Ixidron, see [this article](https://magic.wizards.com/en/news/feature/double-faced-card-rules)), but *can* enter the battlefield face down as a result of being manifested (see [this article](https://blogs.magicjudges.org/rulestips/2015/01/manifest-and-double-faced-cards/)).

The expected result of an Ixidron entering the battlefield should be that Tergrid, as a double-faced card, is unaffected by Ixidron's ability.

Start by tracing Ixidron's face-down effect and the battlefield handling for modal double-faced cards. Reproduce the scenario with Tergrid and Ixidron, then verify that Tergrid remains unaffected while other eligible permanents are turned face down.

Written by the indexing model from the issue text.