docs: sync .org with implementation for backend-size, read-raw-byte, SIGWINCH

backend-protocol.org / simple.lisp:
- Replace hard-coded 80x24 prose with full 5-step fallback chain
  (MY_TERM env vars → ioctl fd 0 → ioctl stdout → /dev/tty → 80x24)
- Document return-from pattern (or discards secondary values)

modern-backend.org / modern.lisp:
- Replace simple ioctl-only prose with 4-step fallback chain
- Document env-var pre-check and /dev/tty fallback

text-input.org / input.lisp:
- Update read-raw-byte prose: with-pinned-objects/vector-sap
  instead of alien buffer (code was already correct, prose stale)
- Add missing (require :sb-posix) to SIGWINCH handler code block
- Document :sb-posix requirement in prose

org/modern-backend.org

@@ -642,27 +642,58 @@ is responsible for redrawing the full screen after resume.
   (values))
 #+END_SRC
 
-** Backend-size via ioctl
+** Backend-size via ioctl and env vars
 
 *** backend-size
 
-Uses TIOCGWINSZ (=0x5413= = 21523) to query actual terminal dimensions
-from the kernel via =ioctl=. The =alien-sap= wrapper ensures
-compatibility across SBCL versions. Returns (values cols rows).
+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.
 
 #+BEGIN_SRC lisp :tangle ../src/backend/modern.lisp
 (defmethod backend-size ((b modern-backend))
-  (or (ignore-errors
-        (let* ((+tiocgwinsz+ 21523)  ; 0x5413 on Linux
-               (winsize (sb-alien:make-alien sb-alien:unsigned-short 4)))
-          (unwind-protect
-               (progn
-                 (sb-unix:unix-ioctl (sb-sys:fd-stream-fd (backend-output-stream b))
-                                     +tiocgwinsz+
-                                     (sb-alien:alien-sap winsize))
-                 (values (sb-alien:deref winsize 1)  ;; cols
-                         (sb-alien:deref winsize 0))) ;; rows
-            (sb-alien:free-alien winsize))))
+  ;; 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.
+      (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)))
 #+END_SRC