Let users pause the autoplaying demo videos

## Problem

The project page starts 16 muted, looping demo videos automatically, but does not provide a pause or stop mechanism.

At `web@66efcf4f1f8a4aaf5cc937e5b6d5bd9724f3ea04`, every video in the method demo grid uses `autoplay loop muted playsinline preload="metadata"` and none has `controls`. Full media inspection shows that all 16 videos run longer than five seconds (11.83–28.53 seconds).

The existing `prefers-reduced-motion` rules disable CSS animation only; they do not stop HTML video playback.

## Reproduction

1. Open https://bytedance.github.io/TableVerse/#pipeline.
2. Scroll to the method demo grid.
3. Observe the 16 looping videos.
4. Try to pause one using the keyboard or an on-page control.

## Expected behavior

Users should have a keyboard-accessible mechanism to pause or stop each automatically moving demo.

## Actual behavior

The videos loop indefinitely without controls.

## Impact

Users who need to pause moving content cannot do so, and multiple off-screen videos may continue decoding unnecessarily.

## Environment

- Source: `web@66efcf4f1f8a4aaf5cc937e5b6d5bd9724f3ea04`
- Live Pages deployment: https://bytedance.github.io/TableVerse/
- 16/16 videos verified with ffprobe, duration 11.83–28.53 seconds

Start by inspecting the method demo grid in the web source at commit 66efcf4f1f8a4aaf5cc937e5b6d5bd9724f3ea04 and reproduce the behavior on the pipeline page. Identify how the 16 autoplaying videos are rendered, then verify that each has a keyboard-accessible pause or stop mechanism and that users can control the moving content.

Written by the indexing model from the issue text.