Add perft testing

engine/src/bitboard.c

@@ -785,6 +785,26 @@ int get_legal_moves(struct Board *board, struct Move *out) {
     return count;
 }
 
+uint64_t perft(struct Board *board, int depth) {
+    if (depth == 0) return 1;
+
+    struct Move moves[256];
+    int n = get_legal_moves(board, moves);
+
+    if (depth == 1) return (uint64_t) n;
+    
+    uint64_t nodes = 0;
+    for (int i = 0; i < n; ++i) {
+        struct Board after;
+        if (!apply_move_on_copy(board, &after, moves[i])) {
+            // Shouldn't happen with legal moves, but be defensive
+            continue;
+        }
+        nodes += perft(&after, depth - 1);
+    }
+    return nodes;
+}
+
 void print_board(const struct Board *board) {
     const char PIECE_CH[12] = {
         'P','N','B','R','Q','K',