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-client-core/src/lib.rs

@@ -0,0 +1,20 @@
+//! Shared client logic for the doctate dictation system.
+//!
+//! This crate is filesystem- and network-aware (unlike `doctate-common`,
+//! which is DTO-only), but path-agnostic: callers pass concrete paths so
+//! the same code works on Linux desktop, Windows desktop, Android, iOS.
+//!
+//! Modules:
+//! - [`case_store`]: per-case marker files, merge-with-server-snapshot logic
+//! - [`snapshot_cache`]: last successful `/api/oneliners` response, cached
+//!   on disk so the UI has data at cold start
+//! - [`oneliner_poller`]: background task that polls the server and feeds
+//!   the case store
+
+pub mod case_store;
+pub mod oneliner_poller;
+pub mod snapshot_cache;
+
+pub use case_store::{CaseMarker, CaseStore, CaseStoreError};
+pub use oneliner_poller::{OnelinerPoller, PollerConfig};
+pub use snapshot_cache::{CachedSnapshot, SnapshotCache};