feat: add semantic adapter APIs for visual-layer consumers - #91

What

Three additive APIs that Remix (a visual-only layer over naked_ui) needs so Naked can be the sole semantics and behavior owner:

• NakedButton.semanticHint — announced on the same Semantics node as the button role, label, enabled state, and tap/long-press actions. Without it, a consumer that wants a hint must wrap the button in a second Semantics node, which is exactly the duplicate-node pattern a headless library should prevent.
• NakedSelect.semanticValue — a human-readable value for the current selection. Today the trigger announces value?.toString(), which for any non-primitive T reads as Instance of 'Foo'. The trigger keeps the merged button + expanded + value contract because SemanticsRole.comboBox still fails debug semantics checks on stable (Missing checks for role, Missing Combo Box Role Implementation flutter/flutter#172918 — note the TODO in semantics.dart cites #159741, which is closed).
• NakedRadioGroup<T> — a thin wrapper over Flutter's RadioGroup.

Why bring back a radio group?

A NakedRadioGroup existed pre-1.0 and was deliberately dropped when the package hardened for 1.0, delegating grouping to Flutter's RadioGroup. That delegation stands — NakedRadio still requires Flutter's registry. This wrapper only supplies what RadioGroup measurably lacks:

1. A nullable onChanged. Flutter's RadioGroup.onChanged is required, so "disabled group" can't be expressed; consumers each invent a no-op adapter.
2. Group-level enabled. Radios inherit it via a typed NakedRadioGroupScope<T>, matching Flutter's typed RadioGroup.maybeOf<T> registry lookup so nested groups of different value types can't read each other's enabled state.
3. An accessible group label. RadioGroup publishes the SemanticsRole.radioGroup node but accepts no label. The label here is a plain labeled container around Flutter's role node — deliberately not a second radioGroup role node, which would announce the group twice (verified by test: exactly one role node with or without a label).

Tests

• semanticHint lives on the single button node (label + hint + tap action on one node).
• semanticValue announced instead of T.toString() for an Object value.
• NakedRadioGroup: tap selects through group onChanged; onChanged: null disables every radio (semantics isEnabled false); enabled: false blocks selection; label produces exactly one labeled container and exactly one radioGroup role node; no label still leaves Flutter's single role node; nested groups of different types keep enabled state aligned.

flutter analyze clean; 711 tests passed, 3 skipped (Flutter 3.44.0 stable).

Consumer

Remix 1.0.0-beta.4 (visual-only-over-Naked pass) consumes all three via a temporary path override; compiling Remix against published 1.0.0-beta.11 fails on exactly these three APIs and nothing else.