[explorer, position] Make Position.board private to the crate

Export a Position::board() method that returns a reference to the internal Board.

position/src/position.rs

@@ -24,7 +24,7 @@ use std::{collections::HashSet, fmt, sync::Arc};
 #[must_use]
 #[derive(Clone, Debug, Default, Eq)]
 pub struct Position {
-    pub board: Board,
+    pub(crate) board: Board,
     pub(crate) moves: Vec<MoveRecord>,
     pub(crate) captures: CapturesList,
 
@@ -48,6 +48,11 @@ impl Position {
             ..Default::default()
         }
     }
+
+    #[must_use]
+    pub fn board(&self) -> &Board {
+        &self.board
+    }
 }
 
 impl Position {