Refactor promoted-widget persisted schema to named variants

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

Current state:
- `src/core/schemas/promotionSchema.ts:20` persists promoted-widget identity plus inline value as a 2/3/4-tuple union.
- `src/core/schemas/promotionSchema.ts:57` exports `getProxyWidgetInlineState()` so callers can inspect tuple arity.
- `src/lib/litegraph/src/subgraph/SubgraphNode.ts:1191` branches on the optional inline tuple state during configure.

Goal:
- Replace the tuple-arity persisted shape with named object variants, preferably a discriminated union.
- Keep migration/tolerant reads for the current tuple forms.
- Remove tuple-position-specific helper surface once callers can narrow by discriminator.

Why:
- The 4-tuple is now a forward persistence commitment. Named variants would make future schema changes additive and easier for extension authors to understand.

┆Issue is synchronized with this [Notion page](https://app.notion.com/p/Issue-11738-Refactor-promoted-widget-persisted-schema-to-named-variants-3516d73d3650812f88d8f8ff48b099e8) by [Unito](https://www.unito.io)

Start with src/core/schemas/promotionSchema.ts at lines 20 and 57, then trace the configure branch in src/lib/litegraph/src/subgraph/SubgraphNode.ts:1191. Define the named persisted variants while retaining tolerant reads for the existing tuple forms, and remove the tuple-position-specific helper once callers can narrow by discriminator.

Written by the indexing model from the issue text.