[moves, position] Implement unmaking moves on a board

Declare an UnmakeMove trait in the moves crate, just like the MakeMove trait from
an earlier commit. Implement this trait for all types that also implement
BoardProvider.

Bring in a whole pile of unit tests from Claude. (Holy shit, using Claude really
saves time on these tests…) Several of these tests failed, and all of those
failures revealed bugs in either MakeMove or UnmakeMove. Huzzah! Include fixes for
those bugs here.

position/src/position/unmake_move.rs

@@ -1,3 +0,0 @@
-// Eryn Wells <eryn@erynwells.me>
-
-pub enum UnmakeMoveError {}