fix: terminal cursor instead of software-drawn █

Replaced software cursor (draw-text █ every frame) with native terminal
cursor (position-cursor using cursor-move + cursor-style). Terminal handles
blinking natively at 500ms — no redraw needed for cursor updates.

- position-cursor: computed input insertion point from state, calls
  cursor-move + cursor-style (:block :blink t) + cursor-show.
- Called from main loop every frame after (sleep 0.1), outside
  redraw's begin-sync/end-sync. No flicker.

org/channel-tui-main.org

@@ -1067,7 +1067,10 @@ Returns T on success, nil on failure. Does NOT wait or retry."
                                                   (format nil "> ~a" (or filter ""))
                                                   (theme-color :input-prompt) bg-p))
                      (cl-tty.backend:end-sync be))
-                  (sleep 0.1)))
+                  (sleep 0.1)
+                  ;; Show terminal cursor at input position every frame
+                  (unless (st :dialog-stack)
+                    (passepartout.channel-tui:position-cursor be w h))))
                (progn (disconnect-daemon)))))
 #+END_SRC