Add guided reading for visual pages

[mokhov]

Problem

Dense visual pages can be difficult to follow on a small screen because the reader must repeatedly zoom, pan, and infer the intended order.

Proposed behavior

Offer guided movement through detected or manually defined panels and speech areas without adding a new file-format requirement.

Acceptance criteria

• Guided regions can be detected automatically or corrected manually.
• Left-to-right, right-to-left, and double-page reading orders are supported.
• Readers can move forward and backward between regions without losing the page position.
• Failure to detect regions leaves the ordinary page completely unchanged.
• Reduced-motion and keyboard-accessible transitions are supported.