fix: data race and overlay bugs (#20)

- redundant ghost text
- prompt deletion bug on overlay
- overlay bug on the non-exists commands

<img width="1152" height="221" alt="image"
src="https://github.com/user-attachments/assets/29ef0c1f-4cd8-4e36-960f-352010fc3bd1"
/>
<img width="1275" height="470" alt="image"
src="https://github.com/user-attachments/assets/aa765110-f9fa-49c8-94f2-767a827f5e5f"
/>
<img width="2517" height="152" alt="image"
src="https://github.com/user-attachments/assets/47a6668f-fb01-46be-9ad2-2c50c283f175"
/>

scripts/iris.zsh

@@ -12,9 +12,14 @@ if [[ -n "$IRIS_FD" ]]; then
     print -u $IRIS_FD -N -r -- "IRIS_CMD_STOP" 2>/dev/null
   }
 
+  _iris_preexec() {
+    print -u $IRIS_FD -N -r -- "IRIS_CMD_START" 2>/dev/null
+  }
+
   autoload -Uz add-zle-hook-widget
   autoload -Uz add-zsh-hook
   
   add-zle-hook-widget line-pre-redraw _iris_send_lbuffer
   add-zsh-hook precmd _iris_precmd
+  add-zsh-hook preexec _iris_preexec
 fi