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/backend-protocol.org

@@ -886,16 +886,14 @@ a hard-coded 80x24 at the end:
 
 *** Backend Write (Simple)
 
-Writes a string to the backend's output stream, forces the stream to
-flush, and returns the length of the string. Uses ~finish-output~ to
-ensure the data is actually sent, which matters for pipe and network
-output.
+Writes a string to the backend's output stream and returns its length.
+Does NOT flush — explicit sync points (~initialize-backend~,
+~end-sync~, etc.) call ~finish-output~ as needed.
 
 #+BEGIN_SRC lisp :tangle ../src/backend/simple.lisp
 (defmethod backend-write ((b simple-backend) string)
   (let ((stream (backend-output-stream b)))
     (write-string string stream)
-    (finish-output stream)
     (length string)))
 #+END_SRC