Implement video/audio ACT rules (12 rules)

## ACT rules requiring media playback

These 12 ACT rules require audio/video playback capabilities which a DOM-only scanner cannot provide.

| ACT Rule | Name |
|---|---|
| `1a02b0` | Audio and visuals of video element have transcript |
| `1ea59c` | Video element visual content has audio description |
| `1ec09b` | Video element visual content has strict accessible alternative |
| `4c31df` | Audio or video element that plays automatically has a control mechanism |
| `aaa1bf` | Audio or video element that plays automatically has no audio > 3 seconds |
| `c3232f` | Video element visual-only content has accessible alternative |
| `c5a4ea` | Video element visual content has accessible alternative |
| `d7ba54` | Video element visual-only content has audio track alternative |
| `e7aa44` | Audio element content has text alternative |
| `ee13b5` | Video element visual-only content has transcript |
| `efbfc7` | Text content that changes automatically can be paused/stopped/hidden |
| `f51b46` | Video element auditory content has captions |

## Blocker
These rules require actually playing media to determine if transcripts, captions, or audio descriptions exist. A DOM-only scanner can at most check for the presence of `` elements but cannot verify content quality.

Related to #350.

Start by reviewing related issue #350 and the DOM-only scanner's handling of media elements, including its possible checks. Determine what playback capabilities are required for the 12 listed ACT rules and define completion criteria for each rule before implementation; no files or tests are identified in this issue.

Written by the indexing model from the issue text.