fix!: resolve SelectItem barrel export collision in favor of the component

[web-padawan]

Problem

Since #390 added the SelectItem React component, the package barrel exports the name SelectItem twice via export * chains:

• export * from "./SelectItem.js" → the new React component (value)
• export * from "./Select.js" → export * from "@vaadin/select/vaadin-select.js" → the deprecated type SelectItem = SelectItemData alias (deprecated in feat: make select-item and select-list-box public, add SelectItemData web-components#12132)

TypeScript resolves the duplicated name to the first star export, so SelectItem in type position now fails in consumer projects:

error TS2749: 'SelectItem' refers to a value, but is being used as a type here.

Resolution was also order-dependent (barrel line order follows schema-discovery order), so a reordering could have silently broken the component import instead. The repo's own validate:types never catches this because the barrel is emitted directly as index.d.ts/index.js and never compiled as source.

Change

• The generated barrel now emits an explicit export { X } from './X.js'; per component after the star exports. Explicit exports beat star exports per-name, so each component name deterministically resolves to the component, independent of barrel order.
• Added a type-level regression fixture (test/typings/SelectItem.tsx, compiled by validate:types) asserting: SelectItem works as a JSX component from the barrel, SelectItemData works in type position, SelectItem in type position is an error, and the deprecated type remains importable from @vaadin/react-components/Select.js.

SelectItemData was already re-exported transitively and keeps working unchanged; docs examples were migrated in vaadin/docs#5831. Runtime is unaffected — the collision was type-only.

BREAKING CHANGE

The deprecated TypeScript type SelectItem is no longer exported from the package barrel; use SelectItemData instead, or import the deprecated type from @vaadin/react-components/Select.js. SelectItem now refers to the React component. (In practice this break already shipped with 25.3.0-alpha6, where the component shadowed the type — this change makes the resolution deterministic and documented.)

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