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.

doctate-common/src/lib.rs

@@ -1,5 +1,6 @@
 pub mod ack;
 pub mod constants;
+pub mod oneliners;
 pub mod timestamp;
 
 pub use ack::{AckResponse, AckStatus};
@@ -7,4 +8,5 @@ pub use constants::{
     API_KEY_HEADER, CONTENT_TYPE_AUDIO_MP4, FIELD_AUDIO, FIELD_CASE_ID, FIELD_RECORDED_AT,
     UPLOAD_PATH,
 };
+pub use oneliners::{OnelinerEntry, OnelinersResponse, ONELINERS_PATH};
 pub use timestamp::{filename_stem_to_recorded_at, now_rfc3339, recorded_at_to_filename_stem};