## Why
When a turn uses multiple environments, the environment context lists each one but did not identify which environment is primary.
## What changed
- Add a `primary` attribute to each environment in multi-environment model context.
- Persist primary selection in world-state snapshots and emit updates when the primary environment changes.
- Restate environments when transitioning between single- and multi-environment context, while preserving the existing single-environment format.
## Testing
- Cover primary-environment changes, single/multi-environment transitions, legacy snapshot compatibility, and deferred-executor promotion after environment startup.
GitOrigin-RevId: 73a1d794a227edef6e9b7844e7f947b7f598dc80
sayan-oai
0a6616f4cf