fix: solid amber █ cursor with finish-output flush

Replaced reverse-video cursor with a solid █ block in :input-prompt
color (amber). Drawn at the insertion point every frame from redraw
and main loop. finish-output ensures the cursor escape reaches the
terminal immediately.

org/channel-tui-view.org

@@ -320,8 +320,7 @@ Returns a list of strings, one per line."
     (setf (st :dirty) (list nil nil nil))))
 
 (defun position-cursor (fb w h)
-  "Reverse-video cursor (Emacs style): shows the character at cursor position
-with foreground and background colors swapped."
+  "Draw a solid block cursor █ at the input insertion point."
   (let* ((sw (if (sidebar-visible-p w) (or (st :sidebar-width) 42) 0))
          (cw (- w sw))
          (hpad 2)
@@ -330,11 +329,9 @@ with foreground and background colors swapped."
          (prompt-w (- cw (* 2 hpad) 2))
          (display-start (max 0 (- pos (1- prompt-w))))
          (cx (+ hpad 2 (- pos display-start)))
-         (cy (- h 6))
-         (ch (if (< pos (length text)) (char text pos) #\Space)))
-    (cl-tty.backend:draw-text fb cx cy (string ch)
-                               (theme-color :bg)         ;; fg = global bg (black)
-                               (theme-color :input-fg)))) ;; bg = text color (reverse)
+         (cy (- h 6)))
+    (cl-tty.backend:draw-text fb cx cy "█" (theme-color :input-prompt) nil)
+    (finish-output (cl-tty.backend::backend-output-stream fb))))
 #+END_SRC
 
 * Implementation — v0.7.0 additions