NakedButton: no way to announce a loading state (live region)

[leoafarias]

Problem

Remix's RemixButton / RemixIconButton previously wrapped NakedButton in their own Semantics(liveRegion: loading, ...) node so screen readers announced entering the loading state. The visual-only-over-Naked pass removed that wrapper — correctly, because it republished the accessible name on a second node — but NakedButton offers no replacement, so entering loading is now silent.

What's needed

A way for a consumer to mark the button's own Semantics node as a live region (or otherwise announce a state change) without wrapping it in a second node. Options:

1. NakedButton.semanticLiveRegion: bool forwarded to the existing single Semantics node (simplest; mirrors semanticHint from feat: add semantic adapter APIs for visual-layer consumers #91).
2. A more general announcement hook, if a bool on one widget is too narrow.

Context

• The single-node contract is tested in naked_button_semantics_test.dart ("exactly one button node"); any fix must keep that invariant.
• Remix 1.0.0-beta.4 documents the lost announcement as a BREAKING change pending this API.