Board Break: a hard slam can snap the board

A slam can break the local skater's board at the nose, in the middle or at
the tail. Half the time the grip tape holds the pieces together and the board
folds or an end hangs; otherwise a piece comes off, flies and lies where it
lands. The board is whole again in hand, on riding and on a respawn.

The pieces are four stretches of the deck in a content mod's copy of the
game's one deck model, each on a spare bone of the board's rig. The boards'
render pose hook only moves bones, because the replay records where a bone is
and not its size: a broken board's deck goes away and its stretches go to
where the pieces lie, and a whole board's stretches go away.

The deck the player chose loads first and other mods that bring a deck model
are set aside for the run, without touching mods.json. Another deck is chosen
while the game runs, through a live apply of the mods.

contrib/board-break.md says how it works and how a deck pack is made.

Extension/Console/commands.cpp

@@ -227,6 +227,7 @@ const Commands &game_commands() {
         register_trainer_commands(*result);
         register_hall_of_meat_commands(*result);
         register_road_rash_commands(*result);
+        register_board_break_commands(*result);
         return result;
     }();
     return *registry;