[progress][meter] SSR hydration mismatch on `aria-valuetext` due to runtime-locale `Intl.NumberFormat`

```

The server (Bun/Node) renders `"30%"` while a browser with a locale that uses NBSP before `%` (e.g. `fr-FR`, `nl-NL`) renders `"30 %"`. The two inputs to `Intl.NumberFormat` are identical — only the default runtime locale differs.

Source: `packages/react/src/utils/formatNumber.ts` — `formatNumberValue(value, locale, format)` falls through to `Intl.NumberFormat(undefined, { style: 'percent' })` when no `locale` or `format` is provided.

The same `formattedValue` is also exposed via `ProgressRootContext` / `MeterRootContext` to `Progress.Value` / `Meter.Value`, so the visible text can mismatch too when those subcomponents are used.

## Expected behavior

`Progress.Root` and `Meter.Root` should render the same `aria-valuetext` on the server and the client out of the box — either by producing a locale-independent default, or by deferring locale-sensitive formatting until after hydration.

## Reproducible example

Minimal repro (any SSR framework — reproduced against TanStack Start on Bun, same outcome on Next.js App Router):

```tsx
import { Progress } from '@base-ui/react/progress'

export default function Page() {
return (





)
}
```

Trigger by running the server with a locale that formats percents without NBSP (e.g. `LANG=C` or `en-US`) and opening the page in a browser with one that does (e.g. `fr-FR`, `nl-NL`, `de-DE`). Console shows the hydration mismatch on `aria-valuetext`.

## Base UI version

v1.3.0

## Which browser are you using?

Chrome (reproduces on any browser whose locale differs from the server's default locale)

## Which OS are you using?

macOS (server); any OS on the client

## Which assistive tech are you using (if applicable)?

N/A — bug surfaces as a React hydration warning; screen reader impact is a brief mismatch until React discards the server tree.

## Additional context

**Root cause** — `packages/react/src/utils/formatNumber.ts`:

```ts
export function formatNumberValue(value, locale, format) {
if (value == null) return ''
if (!format) {
return formatNumber(value / 100, locale, { style: 'percent' })
}
return formatNumber(value, locale, format)
}
```

When neither `locale` nor `format` is passed by the consumer, `Intl.NumberFormat` uses the runtime default locale. This is non-deterministic across SSR boundaries.

`Slider` likely has the same problem via the same utility, though I haven't reproduced it — flagging for triage.

**Possible fixes** (happy to send a PR once the preferred direction is decided):

1. **Locale-independent default** — when `locale` and `format` are both unset, skip `Intl.NumberFormat` and let the existing `getDefaultAriaValueText` fallback (`${value}%`) kick in. Breaking for consumers relying on localized `Progress.Value` output without passing `locale`.
2. **Mount-gated formatting** — render a deterministic fallback on the first render, swap in the localized value after mount (`useEffect`). Non-breaking, one extra render.
3. **`suppressHydrationWarning`** on the root — matches the approach in #4482. Silences the warning but screen readers may still read a mismatched value before hydration completes.

**Consumer workaround** — pass an explicit `locale` to `Progress.Root` / `Meter.Root`:

```tsx

```

This works because both server and client now use the same formatter input.