Introduces a new crate `doctate-client-core` to house shared client-side
logic. This includes:
- `case_store`: Manages local case marker files and merging with server
snapshots.
- `snapshot_cache`: Placeholder for caching server responses.
- `oneliner_poller`: Placeholder for the background polling task.
The workspace configuration and `Cargo.lock` have been updated to
include the new crate.
