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-view.org

@@ -326,6 +326,22 @@ Returns a list of strings, one per line."
       (view-sidebar fb w h))
     (cl-tty.backend:end-sync fb)
     (setf (st :dirty) (list nil nil nil))))
+
+(defun position-cursor (fb w h)
+  "Position terminal cursor at the input insertion point.
+Terminal handles the blinking — we just set position and style once per frame."
+  (let* ((sw (if (sidebar-visible-p w) (or (st :sidebar-width) 42) 0))
+         (cw (- w sw))
+         (hpad 2)
+         (text (input-string))
+         (pos (or (st :cursor-pos) 0))
+         (prompt-w (- cw (* 2 hpad) 2))
+         (display-start (max 0 (- pos (1- prompt-w))))
+         (cx (+ hpad 2 (- pos display-start)))
+         (cy (- h 6)))
+    (cl-tty.backend:cursor-move fb cx cy)
+    (cl-tty.backend:cursor-style fb :block :blink t)
+    (cl-tty.backend:cursor-show fb)))
 #+END_SRC
 
 * Implementation — v0.7.0 additions