feat: Add doctate-client-core crate

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.

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = ["server", "doctate-common", "client-desktop"]
+members = ["server", "doctate-common", "doctate-client-core", "client-desktop"]
 resolver = "3"
 
 [workspace.package]