Keep better track of set of players that know about a face down permanent's real card.

This is more a convenience feature than strictly required.
But once a player knows about a face down's front face, that should stay known until the permanent is no longer face down.

A few situations:
- A player using an effect to look at a face down card (e.g. [[Smoke Teller]] or [[Keeper of the Lens]]) should keep the real info even when that effect end.
- If a face down is result of an effect from a public space (e.g. [[Ixidron]]), all players should be aware of that card. Manifest from top of library when under a "Reveal your top card" is another example.
- Control change (e.g. [[Control Magic]]) should keep the current set of players knowing the real card, and just add the new controller to that set.

No files or tests are named. Start by tracing how face-down permanents record which players know their front face, then examine the Smoke Teller, Keeper of the Lens, Ixidron, manifest, and control-change cases described here. Done means knowledge persists after a temporary look effect, is shared when revealed from public information, and survives control changes while the permanent remains face down.

Written by the indexing model from the issue text.