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
