## What changed
Add `ContextWindowId` as a string-backed protocol type with UUIDv7 generation and parsing. Use it throughout compaction state, rollout reconstruction, token budget context, and request metadata in place of raw UUIDs, preserving the serialized string format and UUIDv7 validation.
GitOrigin-RevId: 786b74fcc331707ca5d6765f429b6e5e322d9328
