feat(sdui): standardize on_change payload to { id, value } across all form input components

[roblevintennis]

Context

Part of the adaptive questionnaire design tracked in #459. For server-side transition logic to work, the consumer must be able to accumulate a complete answers object as the user progresses through a form. This requires every form input component to reliably emit { id, value } when its value changes.

Currently only AgSelectionCardGroup passes a value through on_change. All other form inputs fire the alias with no payload.

Scope

Standardize on_change to emit { id, value } in the React, Vue, and Lit renderers for:

• AgInput
• AgCheckbox
• AgToggle
• AgSelect
• AgRadio
• AgSelectionButton / AgSelectionButtonGroup
• AgSelectionCard / AgSelectionCardGroup (already implemented — verify shape is consistent)

Acceptance criteria

• All listed components emit { id, value } through their on_change dispatch in all three renderers
• SPECIFICATION.md section 5 updated with the standardized payload shape
• Existing action dispatch tests (if any) updated or added to cover payload shape

Related

• design(sdui): adaptive questionnaire flows via server-side transition logic #459 — parent design issue
• SPECIFICATION.md section 5 — Action Dispatch Semantics
• v2/sdui/renderers/ — React, Vue, Lit renderer implementations