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/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "doctate-client-core"
+version.workspace = true
+edition.workspace = true
+description = "Shared client logic for the doctate medical dictation system: case store, oneliner poller, snapshot cache"
+
+[dependencies]
+doctate-common = { path = "../doctate-common" }
+tokio = { workspace = true }
+reqwest = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+uuid = { workspace = true }
+time = { workspace = true }
+tracing = { workspace = true }
+thiserror = "1"
+
+[dev-dependencies]
+tempfile = "3"
+tokio = { workspace = true, features = ["test-util"] }
+wiremock = "0.6"