Implement CSS/layout/interaction ACT rules (5 rules)

## ACT rules requiring CSS computation, layout, or keyboard interaction

| ACT Rule | Name | Blocker |
|---|---|---|
| `59br37` | Zoomed text node is not clipped with CSS overflow | Needs layout/CSS |
| `80af7b` | Focusable element has no keyboard trap | Needs keyboard interaction |
| `oj04fd` | Element in sequential focus order has visible focus | Needs computed styles |
| `ffbc54` | No keyboard shortcut uses only printable characters | Needs keyboard interaction |
| `9bd38c` | Content has alternative for visual reference | Needs visual context |

## Blocker
These rules require capabilities beyond DOM inspection: computed style resolution, layout calculation, or simulating keyboard interaction.

Related to #350.

Start by reviewing related issue #350 and the five ACT rule identifiers in the table. Identify how the scanner currently handles DOM inspection, then scope support for computed styles, layout, visual context, and keyboard interaction. Done means all five listed rules are implemented.

Written by the indexing model from the issue text.