New format: Battle Box

Would adding the Battle Box format (also sometimes known as "Danger Room") to XMage be a feasible possibility, or would it be more complicated than what it seems on the surface? It's a casual format that consists of playing from a a shared library made from a large pile of curated cards. The format removes the issue of being mana screwed or flooded by having only nonland cards in the library and each player starts with 10 lands in exile (5 basics + 5 "enters the battlefield tapped" dual lands), being able to play one of them per turn as if they were in their hand. It's somewhat similar to playing Cube, but without the the need to spend time drafting and building decks, instead all you need is a pre-made pile of a few hundred cards (your battle box) and just play, also ensuring a balanced match-up, since there's no netdecking. The deck is always as good or bad to everyone as the built battle box, and makes in-game decisions matter a lot more.

To get to the point of what would need to be done to add it as a new format option on XMage:

- The host of the game is the only one that needs to provide a deck file, similar to selecting a Cube. There should be no need to apply restrictions to size, number of same name cards, or any ban lists.

- Once the game starts all players share the same library (the deck file the host provided).

- Starting hand size of 6 and no mulligans.

- If the library runs out of cards you do not lose.

- Each player starts the game with 10 lands in exile/command zone and can play one of them from there per turn as if they were in their hand.
1 Plains, 1 Island, 1 Swamp, 1 Mountain, 1 Forest
1 [Meandering River](https://img.scryfall.com/cards/large/front/f/4/f47ee724-da0f-4eb1-b07b-b07e04e9f5b3.jpg?1562305265), 1 [Submerged Boneyard](https://img.scryfall.com/cards/large/front/d/f/df8c56fa-fae6-48ba-813d-0d971b640896.jpg?1562304794), 1 [Cinder Barrens](https://img.scryfall.com/cards/large/front/d/b/db41b554-2bb1-4f11-be29-233d36cc955a.jpg?1562304773), 1 [Timber Gorge](https://img.scryfall.com/cards/large/front/0/7/07076412-18fe-4e15-bdb5-17111b4a66db.jpg?1562300278), 1 [Tranquil Expanse](https://img.scryfall.com/cards/large/front/7/8/78b33867-5ccf-49a1-8e9b-9d2ddac78f17.jpg?1562302863)

- No need for sideboards, games are best-of-1.

Those should be the things needed in order to make it work, as far as I can think. The one big thing I can see being a problem for implementation could be the matter of card ownership during the game due to the shared library mechanic. In paper the rule is usually that once you draw a card you own it until the end of the game or until it is placed back in the library, but that, if indeed a problem for implementation, might make it seem like too much of a headache to implement for anyone to consider doing it, which is what worries me. Alternatively, there's groups who choose, instead of playing from a shared library, to randomly split the box into individual decks of 60 or 40 cards at the start of the game. This would probably be an easier option, but of course, it changes the dynamic of play quite a bit. This alternative would also probably warrant the inclusion of loss by mill again.

Here's a couple of links that do a better job of talking about the format than I could here, for anyone interested:
https://www.channelfireball.com/all-strategy/articles/stark-reality-battle-box/
https://mtgbattlebox.com/rules/
https://www.cardmarket.com/en/Magic/Insight/Articles/Battle-Box-Instant-Magic--Just-Add-Lands-And-Go

There's many more sources on the subject too, if anyone wishes to Google about them.

Again, it's a very casual (and a ton of fun) format, with no official support, so rules can vary from group to group, but the above things should allow it to work in order to be playable on XMage, which I and a good few other people would very much enjoy doing, and would be very grateful if someone was willing to add it.

Thoughts?

The issue specifies Battle Box rules and several possible interpretations, but names no source files, tests, or entry points. First review how XMage represents existing formats and shared libraries, then confirm the ownership, mulligan, land-zone, and deck-handling rules with maintainers. Done means the format is selectable and playable with the listed behavior.

Written by the indexing model from the issue text.