Remove prevLine and fix skip demo wiring

[han4wluc]

Summary

• remove the prevLine action and collapse active pointer handling to a read-only model
• switch the complete VT demo back control to rollbackByOffset: { offset: -1 }
• fix the skip demo to use the supported setSkipUnseenText action and add runtime rendering coverage for its boolean payload

Testing

• bun run vitest run
• bun run vt:generate
• browser-level VT verification against candidate/complete/one.html confirming the rendered Back control dispatches rollbackByOffset: { offset: -1 } and moves the pointer from line2 back to line1 when routed through window.__vtHandleRouteGraphicsEvent(...)