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/backend-protocol.org

@@ -817,14 +817,71 @@ No-op — simple backend has no terminal state to restore.
 
 *** Backend Size (Simple)
 
-Returns hard-coded 80x24 dimensions. A real implementation would use
-ioctl or TIOCGWINSZ, but the simple backend avoids OS-specific calls
-for maximum portability.
+Queries actual terminal dimensions through a fallback chain, with
+a hard-coded 80x24 at the end:
+
+1. **Env var pre-check** — ~MY_TERM_COLS~ / ~MY_TERM_ROWS~, set by the
+   calling script before ~exec sbcl~. Checked with ~return-from~ so that
+   /both/ values are preserved (Common Lisp's ~or~ discards secondary
+   values).
+2. **ioctl on fd 0 (stdin)** — the parent's real terminal fd.
+3. **ioctl on stdout** — fast and correct after SIGWINCH at runtime.
+4. **ioctl on ~/dev/tty~** — fallback when stdin/stdout are pipes.
+5. **~(values 80 24)~** — last resort.
 
 #+BEGIN_SRC lisp :tangle ../src/backend/simple.lisp
 (defmethod backend-size ((b simple-backend))
-  ;; Try ioctl, fall back to 80x24
-  (values 80 24))
+  ;; MY_TERM_COLS/MY_TERM_ROWS — set by the calling script.
+  ;; Check with return-from to preserve both values.
+  (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 ;; ioctl on fd 0 (stdin) — the parent's own terminal.
+      (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 0 21523
+                                                 (sb-alien:alien-sap winsize))))
+                     (when ok
+                       (let ((c (sb-alien:deref winsize 1))
+                             (r (sb-alien:deref winsize 0)))
+                         (when (and c r (> c 0) (> r 0))
+                           (values c r)))))
+                (sb-alien:free-alien winsize))))
+          (when (and cols rows (> cols 0) (> rows 0))
+            (values cols rows)))
+      ;; ioctl on stdout fd — fast, correct after SIGWINCH at runtime.
+      (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)
+                               (sb-alien:deref winsize 0))))
+                (sb-alien:free-alien winsize))))
+          (when (and cols rows (> cols 0) (> rows 0))
+            (values 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
+                       (values (sb-alien:deref winsize 1)
+                               (sb-alien:deref winsize 0))))
+                   (sb-alien:free-alien winsize))
+              (sb-unix:unix-close tty-fd)))))
+      (values 80 24)))
 #+END_SRC
 
 *** Backend Write (Simple)