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.lock

@@ -1145,6 +1145,23 @@ dependencies = [
  "libloading 0.8.9",
 ]
 
+[[package]]
+name = "doctate-client-core"
+version = "0.1.0"
+dependencies = [
+ "doctate-common",
+ "reqwest",
+ "serde",
+ "serde_json",
+ "tempfile",
+ "thiserror 1.0.69",
+ "time",
+ "tokio",
+ "tracing",
+ "uuid",
+ "wiremock",
+]
+
 [[package]]
 name = "doctate-common"
 version = "0.1.0"