fix: read-raw-byte checks poll result before unix-read

The original code called unix-simple-poll then unconditionally
called unix-read, ignoring the poll result. When poll returned
nil (no data), unix-read would block indefinitely. Fixed by
checking poll result: only read if poll says data is ready.

src/components/input.lisp

@@ -74,9 +74,11 @@
     (sb-sys:with-pinned-objects (buf)
       (let ((sap (sb-sys:vector-sap buf)))
         (if timeout-ms
-            (progn (sb-unix:unix-simple-poll fd :input timeout-ms)
+            ;; Poll first: only read if data is ready
+            (if (sb-unix:unix-simple-poll fd :input timeout-ms)
                 (let ((n (sb-unix:unix-read fd sap 1)))
-                     (if (= n 1) (aref buf 0) (values nil :eof))))
+                  (if (= n 1) (aref buf 0) (values nil :eof)))
+                nil)
             (let ((n (sb-unix:unix-read fd sap 1)))
               (if (= n 1) (aref buf 0) (values nil :eof))))))))