GHSA-fm7p-gw32-828p

### Summary

Despite the 0.104.0 patch escaping attribute-bearing constructs (`\htmlData`, `\href`), text-content reflection was missed. The `\text{}`, `\mbox{}` commands accept arbitrary characters in their body and emit them raw and unescaped into both the HTML markup and the MathML output, leading to XSS.

### Details

`Box.toMarkup` at `src/core/box.ts:356` concatenates `this.value` into the rendered span without HTML-escaping. In text mode any literal character (`<`, `>`, `&`, `"`) is wrapped into a `TextAtom` whose `value` is the raw codepoint and lands in the markup unchanged. The MathML serializer at `src/formats/atom-to-math-ml.ts` is independently broken: `xmlEscape` deliberately omits the `&` rule, and `scanText`, `case 'text'`, and the `mode === 'text'` early return all emit `atom.value` raw.

Both outputs flow into `innerHTML` sinks via the public API. `<math-span>` / `<math-div>` (`src/public/math-static-elements.ts:331,407`) bypass `MathfieldElement.createHTML` entirely. The editor and SSR paths route through `createHTML`, but its default value is the identity function (`src/public/mathfield-element.ts:789`).

### PoC

1. Go to https://mathlive.io/mathfield/demo/ 2. open DevTools console and paste:

```js const s = document.createElement('math-span'); s.style.display = 'block'; s.textContent = '\\text{<img src=x onerror=alert(1)>}'; document.body.appendChild(s); s.scrollIntoView(); ```

Equivalent payloads: `\mbox{<img src=x onerror=alert(1)>}`

or

```js import { convertLatexToMarkup } from 'mathlive'; document.body.innerHTML = convertLatexToMarkup('\\text{<img src=x onerror=alert(1)>}'); ```

### Impact

MathLive users who render untrusted mathematical expressions can encounter malicious input that runs arbitrary JavaScript.