[tabs] Consider generic value typing for Root

# Feature request

## Summary

`Tabs.Root` currently uses `Tabs.Tab.Value` for `value`, `defaultValue`, and `onValueChange`. Since `Tabs.Tab.Value` is `any | null`, these props effectively use `any` and lose their shared value type.

A Root-only generic could preserve that relationship:

```ts
interface TabsRootProps {
value?: Value | null;
defaultValue?: Value | null;
onValueChange?: (
value: Value | null,
eventDetails: TabsRoot.ChangeEventDetails,
) => void;
}
```

This would support inference and explicit value types without attempting to propagate the generic to `Tabs.Tab` or `Tabs.Panel`, which TypeScript cannot automatically connect to the parent generic.

One tradeoff is nullability. Uncontrolled Tabs can notify `null` when no enabled fallback exists, while controlled Tabs do not perform these automatic fallback updates. Should `onValueChange` use a uniform `Value | null` type, or should controlled and uncontrolled props be typed separately?

## Examples in other libraries

Base UI already uses Root-only generic value typing with `Value | null` for [`NavigationMenu.Root`](https://github.com/mui/base-ui/pull/4328).

Related discussions:

- https://github.com/mui/base-ui/issues/81#issuecomment-2166741362
- https://github.com/mui/base-ui/issues/1076

## Motivation

Consumers commonly use string unions or enums for Tabs state. Generic Root typing would preserve that domain across `value`, `defaultValue`, and `onValueChange` instead of losing it to `any`.

I would like to confirm whether a Root-only generic with a nullable callback is the preferred direction.

Start at the Tabs.Root props and trace how value, defaultValue, and onValueChange are currently typed. Compare the proposed Root-only generic and nullable callback with the linked NavigationMenu.Root approach and related discussions; done means the generic and controlled versus uncontrolled nullability direction is agreed and reflected in the component types.

Written by the indexing model from the issue text.