[dialog] Closing a nested Dialog dismisses its parent inside a closed shadow root

### Summary

When the app is rendered inside a **closed** shadow root, closing a nested `Dialog` also dismisses its parent `Dialog` with `reason: "outside-press"`.

Version: `@base-ui/react@1.7.0` (nothing in the 1.8.0 changelog looks related). Chrome 152.

### Steps to reproduce

1. Render the app inside `host.attachShadow({ mode: "closed" })`.
2. Open a modal `Dialog`, and from inside its `Popup` open a second `Dialog`.
3. Click the nested dialog's `Dialog.Close` button (or the close button in its popup).

**Expected:** only the nested dialog closes.
**Actual:** both dialogs close. The parent's `onOpenChange` fires with `reason: "outside-press"` and `event.type === "click"`.

Escape and clicking the backdrop behave correctly — only a click that lands *inside* the nested dialog reproduces it.

```jsx
const host = document.createElement("div")
document.body.append(host)
createRoot(host.attachShadow({ mode: "closed" })).render()

function App() {
const [outer, setOuter] = React.useState(true)
const [inner, setInner] = React.useState(false)
return (




setInner(true)}>open nested




close






)
}
```

Switching only `mode: "closed"` to `mode: "open"` makes the problem disappear.

### What I found

`useDismiss` registers its outside-press listeners on `ownerDocument(floatingElement)`. From a listener outside the shadow tree, a **closed** shadow root truncates `event.composedPath()`, so `getTarget(event)` (`internals/shadowDom.js`) returns the shadow **host** rather than the clicked element.

Two consequences for the parent dialog:

- `isEventTargetWithin(event, floating)` is `composedPath().includes(node)`, which can never be true — the "press was inside" checks cannot fire.
- In the dialog's `outsidePress` predicate (`dialog/root/useDialogRoot.js`), the fallback `contains(target, popupElement)` is true, because the host *is* an ancestor of every popup. So the press is classified as outside.

The remaining guard is `isTopmost` (`ownNestedOpenDialogs === 0`). It does not hold here because `closeOnPressOutsideCapture` defers the decision via `addTargetEventListenerOnce`, and that listener is attached to the retargeted host. The host is an ancestor of the React root inside the shadow tree, so by the time it runs, React has already handled the click, closed the nested dialog, and reset the parent's `ownNestedOpenDialogs` to `0` — the parent is topmost again and dismisses itself.

A `FloatingTree`-based check would not help either, since `isEventWithinFloatingTree` also relies on `composedPath()`.

Reproduce the nested Dialog case in a closed shadow root, then trace useDismiss, internals/shadowDom.js, and dialog/root/useDialogRoot.js, focusing on target retargeting and the deferred closeOnPressOutsideCapture listener. Verify the fix by confirming that clicking the nested Dialog.Close closes only the nested dialog while Escape and backdrop clicks remain correct.

Written by the indexing model from the issue text.