Add type definition for Math.sumPrecise (ES2025 / TC39 proposal)

Nobody has claimed this yet.

---

### Bug report

#### 📝 Description

`Math.sumPrecise` is part of the [TC39 `proposal-math-sum`](https://github.com/tc39/proposal-math-sum) which reached **Stage 4 on 2025-07-28**, making it part of **ES2026** (not ES2025 as originally stated — the proposal missed the ES2025 cutoff by about one month). Major runtimes like **Bun**, **Firefox 137+**, and **Safari 18.4** have already implemented it, though V8/Node.js has not yet.

However, TypeScript does **not** include type definitions for this API — searching through all `lib.es*.d.ts` files yields no results for `sumPrecise`.

#### 🐛 The problem

This causes a compile error:

```
Property 'sumPrecise' does not exist on type 'Math'. ts(2339)
```

Users are forced to either:
- Use workarounds like type augmentation (`global.d.ts`)
- Fall back to `@ts-expect-error`
- Use incorrect alternatives like `Array.reduce((a, b) => a + b)` which lose precision

#### ✅ Expected behavior

The `interface Math` should be extended in `lib.esnext.math.d.ts`:

```typescript
interface Math {
/**
* Returns the sum of the values in the iterable using a more precise
* summation algorithm than naive floating-point addition.
* Returns `-0` if the iterable is empty.
* @param numbers An iterable (such as an Array) of numbers.
* @throws {TypeError} If `numbers` is not iterable, or if any value in the iterable is not of type `number`.
* @throws {RangeError} If the iterable yields 2^53 or more values.
*/
sumPrecise(numbers: Iterable): number;
}
```

#### 🔗 Related
- TC39 Proposal: https://github.com/tc39/proposal-math-sum
- Stage 4 meeting notes: https://github.com/tc39/notes/blob/main/meetings/2025-07/july-28.md#mathsumprecise-for-stage-4
- MDN Docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sumPrecise
- Firefox 137+: shipped
- Safari 18.4 (JavaScriptCore): shipped
- Bun: shipped
- V8 / Node.js / Chrome: not yet

---

Start in lib.esnext.math.d.ts and compare the declaration needed for Math.sumPrecise with the linked TC39 proposal and existing Math definitions. The work is complete when the Math interface exposes sumPrecise with the documented iterable-of-number signature and TypeScript accepts calls without an augmentation or error suppression.

Written by the indexing model from the issue text.