Design clean public API for extension authors to integrate custom widgets with widgetValueStore

## Context

PR #9230 introduces `useDomValueBridge` to synchronize DOM widget values with the `widgetValueStore` by intercepting `Object.defineProperty` on element.value and listening for input events. While this approach works as a bridge for extension-created DOM widgets, it relies on internal value-descriptor patterns that are not part of a supported public API.

## Problem

Extension authors currently lack a clean, documented integration point to wire custom widgets to the `widgetValueStore`. The current value-descriptor pattern is an implementation detail rather than a stable extension API.

## Proposed Solution

Design and implement a proper public API that:
- Provides a clear, documented way for extension authors to integrate custom widgets with the store
- Replaces the current value-descriptor pattern with a supported integration mechanism
- Maintains backward compatibility where feasible
- Includes documentation and migration guidance

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9230
- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9230#discussion_r2857107802
- Requested by: @christian-byrne

## Acceptance Criteria

- [ ] Public API design documented
- [ ] Extension author documentation created
- [ ] Deprecation path defined for current pattern (if applicable)
- [ ] Implementation plan outlined

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