fix(stdio): correct position tracking on mode switch and free owned buffers on close

include/stdio.h

@@ -206,7 +206,13 @@ fseeko(FILE *stream, off_t offset, int whence);
 long
 ftell(FILE *stream);
 
-__attribute__((pure)) off_t
+/*
+ * Not marked pure: the implementation can allocate the stream buffer on
+ * first use (an observable side effect), so the compiler must not elide
+ * or reorder the call. (feof/ferror/fileno below are genuinely read-only
+ * and keep their pure attribute.)
+ */
+off_t
 ftello(FILE *stream);
 
 /* Rewind to the start and clear feof/ferror (equivalent to