fix: add blocking-read-based CSI 18t terminal size query fallback

%query-terminal-size uses blocking read-char on an fd 0 stream
to read the terminal's response to \033[18t. This works even when
unix-simple-poll on fd 0 returns NIL (unlike read-char-no-hang).
Added as fallback in both modern and simple backends.

src/backend/modern.lisp

@@ -175,6 +175,7 @@ as a fallback when a keyword is not in *named-colors*.")
                  (values (sb-alien:deref winsize 1)  ;; cols
                          (sb-alien:deref winsize 0))) ;; rows
             (sb-alien:free-alien winsize))))
+      (%query-terminal-size)
       (values 80 24)))
 
 (defmethod backend-write ((b modern-backend) string)