Commander swap rule implementation

It would be nice to add the possibility to swap commanders after game 1 and game 2 in Duel Commander games.

400.6. Sideboards
DC doesn’t use sideboards. Players can’t have sideboard cards. The only cards of a DC deck are the 100 cards from the main (and only) deck. The only time allowed to players between rounds is for swapping commanders (see section 400.7 of this document).

400.7. Swapping commanders
400.7.1. Players are allowed to swap commander(s) after each game of the same round. This means that after the first game is played each round only, each player may shuffle his or her commander(s) into his or her deck and present either the same one(s) or new commander(s) from the same deck before each following game begins during each round.
400.7.2. For the first game each round, each player puts their commander(s) from their deck face up into the Command zone before shuffling (see Magic The Gathering Comprehensive Rules at section 103.1b).
400.7.3. After the first game, each player reveals their commanders simultaneously by putting them willingly face up in the Command zone after making choices, or by putting them face down in the Command zone, then having both players turning them face up at the same time before shuffling (see Magic The Gathering Comprehensive Rules at section 101.4a). This action can only be done once per game.
400.7.4. Commanders must remain legal before each game begins. Players can’t change color identity in between games (see Magic The Gathering Comprehensive Rules at section 903.4a), and the choice of Commander(s) has to follow other construction rules defined in section 400.4 of this document (for example: players can’t choose a banned Commander).
400.7.5. Only Commanders that are part of the original deck are legal. If the tournament uses registration deck lists, the commanders must be listed within the deck.
400.7.6. Players are expected to take a decent and short time to choose Commanders in between games.
400.7.7. Players may change the number of Commanders by swapping Commanders with Partner ability and a single Commander without this ability (see Magic The Gathering Comprehensive Rules at section 702.123).

The issue provides Duel Commander rules for swapping commanders but names no files, tests, or entry points. Start by locating the game and deck-validation code related to commander selection; done means all listed swapping, legality, partner, and reveal rules work between games.

Written by the indexing model from the issue text.