fix(autocomplete): fix value persistence in DOM text widgets

Remove multiple sources of truth and async sync chains that caused
values to be lost during load/switch workflow or reload page.

Changes:
- Remove internalValue state variable from main.ts
- Update getValue/setValue to read/write DOM directly via widget.inputEl
- Remove textValue reactive ref and v-model from Vue component
- Remove serializeValue, onSetValue, and watch callbacks
- Register textarea reference on mount, clean up on unmount
- Simplify AutocompleteTextWidgetInterface

Follows ComfyUI built-in addMultilineWidget pattern:
- Single source of truth (DOM element value only)
- Direct sync (no intermediate variables or async chains)

Also adds documentation:
- docs/dom-widgets/value-persistence-best-practices.md
- docs/dom-widgets/README.md
- Update docs/dom_widget_dev_guide.md with reference

vue-widgets/src/main.ts

@@ -423,20 +423,17 @@ function createAutocompleteTextWidgetFactory(
 
   forwardMiddleMouseToCanvas(container)
 
-  let internalValue = ''
-
   const widget = node.addDOMWidget(
     widgetName,
     `AUTOCOMPLETE_TEXT_${modelType.toUpperCase()}`,
     container,
     {
       getValue() {
-        return internalValue
+        return widget.inputEl?.value ?? ''
       },
       setValue(v: string) {
-        internalValue = v ?? ''
-        if (typeof widget.onSetValue === 'function') {
-          widget.onSetValue(v)
+        if (widget.inputEl) {
+          widget.inputEl.value = v ?? ''
         }
       },
       serialize: true,