refactor: move Arena and ArenaMem to memory_defs.h (#26240)

src/nvim/terminal.h

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ -9,7 +8,7 @@ typedef void (*terminal_write_cb)(char *buffer, size_t size, void *data);
 typedef void (*terminal_resize_cb)(uint16_t width, uint16_t height, void *data);
 typedef void (*terminal_close_cb)(void *data);
 
-#include "nvim/buffer_defs.h"
+#include "nvim/buffer_defs.h"  // IWYU pragma: keep
 
 typedef struct {
   void *data;  // PTY process channel