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

[coderabbitai]

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: fix: migrate DOM widgets to ComponentWidgetImpl and add element value bridge #9230
• Discussion: fix: migrate DOM widgets to ComponentWidgetImpl and add element value bridge #9230 (comment)
• 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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