fix: remove per-call finish-output from backend-write (flush once per frame via end-sync)

backend-write flushed output after every single draw-text/draw-rect
call, causing hundreds of individual flushes per frame. This caused
visible flicker on slow terminals.

Remove finish-output from backend-write — all critical flush points
(initialize-backend, shutdown-backend, enable-mouse, enable-bracketed-paste,
end-sync) already call finish-output explicitly.

DECICM sync (begin-sync/end-sync) wraps every frame boundary,
making the frame render atomically with a single flush at end-sync.

org/modern-backend.org

@@ -701,15 +701,15 @@ Uses a fallback chain to determine terminal dimensions:
 
 *** backend-write
 
-Writes a string to the backend's output stream, flushing after each
-write to ensure the terminal receives the escape sequence immediately.
+Writes a string to the backend's output stream. Does NOT flush — the
+caller is responsible for calling ~finish-output~ at appropriate sync
+points (frame boundaries via ~end-sync~, initialization, shutdown).
 Returns the string length for protocol compatibility.
 
 #+BEGIN_SRC lisp :tangle ../src/backend/modern.lisp
 (defmethod backend-write ((b modern-backend) string)
   (let ((stream (backend-output-stream b)))
     (write-string string stream)
-    (finish-output stream)
     (length string)))
 #+END_SRC