## What changed
- Add `SessionModelIdStore` with SQLite and in-memory implementations for looking up and assigning aliases by session, namespace, and external key.
- Commit SQLite assignments before returning them, and recheck under a write transaction so concurrent writers reuse the same alias.
- Support snapshotting and initializing alias maps from generator state and ordered keys, and remove mappings when their thread is deleted.
- Expose alias stores through `ThreadStore` and add distinct types for registered environment IDs and model-facing `env` aliases.
## Testing
Add a test using two SQLite runtime instances to verify that concurrent assignments return the same alias, different keys receive different aliases, and lookups and snapshots preserve the assignments.
GitOrigin-RevId: 2668f605251ca76d4891f70f94bfb4151ba8142c
