Implement rollback timeline model

[han4wluc]

Summary

• replace section-local rollback assumptions with a per-context linear rollback timeline and inclusive checkpoint replay
• restore rollback state from the actions that actually executed at each checkpoint, derive rollback start state from project-defined context defaults, harden replay cleanup with try/finally, and suppress nextLineConfig.auto timers during rollback restore
• move rollback visual coverage into a dedicated Rollback VT section with focused pages for to-line, by-offset, choice-event, cross-section, layered-view restore, auto suppression, and a 20-line multi-section/multi-scene back-one-line sandbox

Validation

• bun run test
• bun run lint
• rtgl vt report --folder rollback