fix: use :input :interactive for stty size subprocess

:input :interactive opens /dev/tty for the child's stdin, giving
stty access to the real terminal for its ioctl query.

src/backend/modern.lisp

@@ -162,12 +162,12 @@ as a fallback when a keyword is not in *named-colors*.")
   (values))
 
 (defmethod backend-size ((b modern-backend))
-  (or ;; stty size with :input :inherit — preserves the parent's fd 0
-      ;; (the terminal) so stty can query it via ioctl.
+  (or ;; stty size with :input :interactive — opens /dev/tty for the
+      ;; child's stdin so stty can query the terminal via ioctl.
       (multiple-value-bind (cols rows)
           (ignore-errors
             (let* ((out (uiop:run-program '("stty" "size") :output :string
-                                          :input :inherit
+                                          :input :interactive
                                           :ignore-error-status t))
                    (parts (and out (uiop:split-string
                                      (string-trim '(#\newline #\space) out)))))

src/backend/simple.lisp

@@ -22,11 +22,11 @@
   (values))
 
 (defmethod backend-size ((b simple-backend))
-  (or ;; stty size with :input :inherit — preserves parent's fd 0.
+  (or ;; stty size with :input :interactive — opens /dev/tty.
       (multiple-value-bind (cols rows)
           (ignore-errors
             (let* ((out (uiop:run-program '("stty" "size") :output :string
-                                          :input :inherit
+                                          :input :interactive
                                           :ignore-error-status t))
                    (parts (and out (uiop:split-string
                                      (string-trim '(#\newline #\space) out)))))