fix: add / env var fallback in backend-size

ioctl on stdout's fd can disagree with the real terminal size when
the process is started with stdout redirected or in some terminal
multiplexer configurations. / are set by every POSIX
shell at process start and reflect the actual terminal dimensions.

Priority: ioctl → env vars → 80x24 fallback.
This covers both initial sizing and dynamic SIGWINCH-driven resize.

src/backend/simple.lisp

@@ -39,6 +39,13 @@
                  (values (sb-alien:deref winsize 1)
                          (sb-alien:deref winsize 0)))
              (sb-alien:free-alien winsize))))
+      (ignore-errors
+        (let ((cols (parse-integer (sb-ext:posix-getenv "COLUMNS")
+                                   :junk-allowed t))
+              (rows (parse-integer (sb-ext:posix-getenv "LINES")
+                                   :junk-allowed t)))
+          (when (and cols rows (> cols 0) (> rows 0))
+            (values cols rows))))
       (values 80 24)))
 
 (defmethod backend-write ((b simple-backend) string)