fix: backend-size returns both cols and rows via multi-value-bind (or discards secondary values)

The OR pattern inside backend-size used (or (multiple-value-bind ...)
...), but multiple-value-bind only returns the primary value of its
body. When the env-var shortcut was removed, both calls to backend-size
(the cols nth-value 0 and rows nth-value 1) returned the same primary
value, making rows always nil.

Restructure with nested multiple-value-bind/values chains so both
return values propagate correctly through all fallback stages.
Also remove MY_TERM_COLS/ROWS env-var pre-check — it returned stale
startup dimensions after terminal resize.

org/modern-backend.org

@@ -642,59 +642,48 @@ is responsible for redrawing the full screen after resume.
   (values))
 #+END_SRC
 
-** Backend-size via ioctl and env vars
+** Backend-size via ioctl
 
 *** backend-size
 
-Uses a fallback chain to determine terminal dimensions:
-
-1. **Env var pre-check** — ~MY_TERM_COLS~ / ~MY_TERM_ROWS~, set by
-   the calling script before ~exec sbcl~. Uses ~return-from~ to preserve
-   both values (~or~ discards secondary values).
-2. **ioctl on stdout** — fast, correct after SIGWINCH at runtime.
-3. **ioctl on ~/dev/tty~** — fallback when stdout is not a terminal.
-4. **~(values 80 24)~** — last resort.
+Uses ioctl (TIOCGWINSZ = 21523) to query actual terminal dimensions
+from the kernel, with a ~/dev/tty~ fallback and 80x24 last resort.
 
 #+BEGIN_SRC lisp :tangle ../src/backend/modern.lisp
 (defmethod backend-size ((b modern-backend))
-  ;; MY_TERM_COLS/MY_TERM_ROWS — set by the calling script.
-  ;; Check FIRST with return-from so both values (cols and rows)
-  ;; are preserved (or discards secondaries).
-  (let ((cstr (sb-ext:posix-getenv "MY_TERM_COLS"))
-        (rstr (sb-ext:posix-getenv "MY_TERM_ROWS")))
-    (when (and cstr rstr)
-      (let ((cols (parse-integer cstr :junk-allowed t))
-            (rows (parse-integer rstr :junk-allowed t)))
-        (when (and cols rows (> cols 0) (> rows 0))
-          (return-from backend-size (values cols rows))))))
-  (or
-      (multiple-value-bind (cols rows)
-          (ignore-errors
-            (let* ((winsize (sb-alien:make-alien sb-alien:unsigned-short 4)))
-              (unwind-protect
-                   (let ((ok (sb-unix:unix-ioctl
-                               (sb-sys:fd-stream-fd (backend-output-stream b))
-                               21523 (sb-alien:alien-sap winsize))))
-                     (when ok
-                       (values (sb-alien:deref winsize 1)  ;; cols
-                               (sb-alien:deref winsize 0)))) ;; rows
-                (sb-alien:free-alien winsize))))
-          (when (and cols rows (> cols 0) (> rows 0))
-            (values cols rows)))
-      ;; Direct ioctl on /dev/tty.
+  ;; Try ioctl on stdout, fall back to /dev/tty, then 80x24.
+  ;; Each arm uses multiple-value-bind/values to preserve both cols and rows
+  ;; (or discards secondary values, so we avoid it for multi-value returns).
+  (multiple-value-bind (cols rows)
       (ignore-errors
-        (let ((tty-fd (sb-unix:unix-open "/dev/tty" 0 0)))
-          (when (and tty-fd (numberp tty-fd) (> tty-fd 0))
-            (unwind-protect
-                 (let* ((winsize (sb-alien:make-alien sb-alien:unsigned-short 4)))
-                   (let ((ok (sb-unix:unix-ioctl tty-fd 21523
-                                                 (sb-alien:alien-sap winsize))))
-                     (when ok
-                       (let ((cols (sb-alien:deref winsize 1))
-                             (rows (sb-alien:deref winsize 0)))
-                         (values cols rows)))))
-              (sb-unix:unix-close tty-fd)))))
-      (values 80 24)))
+        (let* ((winsize (sb-alien:make-alien sb-alien:unsigned-short 4)))
+          (unwind-protect
+               (let ((ok (sb-unix:unix-ioctl
+                            (sb-sys:fd-stream-fd (backend-output-stream b))
+                            21523 (sb-alien:alien-sap winsize))))
+                 (when ok
+                   (values (sb-alien:deref winsize 1)  ;; cols
+                           (sb-alien:deref winsize 0)))) ;; rows
+            (sb-alien:free-alien winsize))))
+    (if (and cols rows (> cols 0) (> rows 0))
+        (values cols rows)
+        ;; Direct ioctl on /dev/tty.
+        (multiple-value-bind (cols rows)
+            (ignore-errors
+              (let ((tty-fd (sb-unix:unix-open "/dev/tty" 0 0)))
+                (when (and tty-fd (numberp tty-fd) (> tty-fd 0))
+                  (unwind-protect
+                       (let* ((winsize (sb-alien:make-alien sb-alien:unsigned-short 4)))
+                         (let ((ok (sb-unix:unix-ioctl tty-fd 21523
+                                                       (sb-alien:alien-sap winsize))))
+                           (when ok
+                             (let ((cols (sb-alien:deref winsize 1))
+                                   (rows (sb-alien:deref winsize 0)))
+                               (values cols rows)))))
+                    (sb-unix:unix-close tty-fd)))))
+          (if (and cols rows (> cols 0) (> rows 0))
+              (values cols rows)
+              (values 80 24))))))
 #+END_SRC
 
 ** Capability query and write