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-state.org

@@ -23,6 +23,7 @@ All state mutation flows through event handlers in the controller.
   (:export :tui-main :st :add-msg :now :input-string
            :queue-event :drain-queue :init-state
             :view-status :view-chat :view-input :redraw
+            :position-cursor
             :input-panel-top
            :on-key :on-daemon-msg :send-daemon
            :connect-daemon :disconnect-daemon