## Why
World-state sections could return only a single optional text fragment, with message separation controlled by the fragment itself. They could not contribute explicit `ResponseItem` values alongside instructions.
## What changed
- Let sections return ordered `WorldStateUpdate` values containing fragments or response items, with `Prefix`, `Standalone`, or `Mergeable` placement.
- Preserve explicit item boundaries when grouping context updates, and place prefix updates first when assembling initial context.
- Move standalone-message decisions from fragments to world-state updates and retain snapshot handling.
## Testing
Add tests for mixed fragment/item ordering, role and standalone boundaries, initial-context prefix placement, and snapshot restoration without repeated updates.
GitOrigin-RevId: c2514c3c98538da83f5140c10a9d89f531de6549