Collapse promoted-widget value resolution surface

Follow-up from #11559 and DrJKL's review.

Current state:
- `src/core/graph/subgraph/promotedWidgetView.ts:380` exposes `getScopedStoreValue()` for serialization.
- `src/core/graph/subgraph/promotedWidgetView.ts:386` uses `getStoreBackedValue()`.
- `src/core/graph/subgraph/promotedWidgetView.ts:404`, `src/core/graph/subgraph/promotedWidgetView.ts:408`, `src/core/graph/subgraph/promotedWidgetView.ts:412`, and `src/core/graph/subgraph/promotedWidgetView.ts:416` split related resolution paths across several near-synonymous helpers.

Goal:
- Collapse promoted-widget value resolution into one typed resolver that names the precedence order.
- Keep callers from needing to know whether the value came from scoped state, host widget fallback, legacy state, or missing state.

Why:
- A single resolver should make promoted-widget value semantics easier to audit before removing transitional fallbacks.

┆Issue is synchronized with this [Notion page](https://app.notion.com/p/Issue-11743-Collapse-promoted-widget-value-resolution-surface-3516d73d3650816697d9f3bee35950b8) by [Unito](https://www.unito.io)