Cap height of slow list demo previews

[TunaDev0]

Summary

Closes issue #8486

• add a max height and scrolling to the .items container in the artificially slow useDeferredValue list demos
• add a max height and scrolling to the .items container in the artificially slow useTransition posts demos
• keep the existing item counts and slowdown behavior unchanged, while preventing the preview output from pushing the rest of the page too far down

Testing

• pre-commit hooks passed during commit creation

Why this better:

1. Names actual change
2. Mentions both pages
3. States no behavior change
4. Matches issue intent closely
   If you want closer to issue wording, use:

Involved routes:

• /reference/react/useDeferredValue
• /reference/react/useTransition

Example of improvement

Left: PR changes
Right: current implementation
The current implementation can expand the list to around 12500px in height.

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

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This PR introduced no changes to the JavaScript bundle! 🙌