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/snapshot_cache.rs

@@ -0,0 +1,24 @@
+//! Placeholder — full implementation follows in next step.
+
+use std::path::PathBuf;
+
+use doctate_common::oneliners::OnelinersResponse;
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CachedSnapshot {
+    pub etag: String,
+    pub fetched_at: String,
+    pub response: OnelinersResponse,
+}
+
+pub struct SnapshotCache {
+    #[allow(dead_code)]
+    path: PathBuf,
+}
+
+impl SnapshotCache {
+    pub fn new(path: PathBuf) -> Self {
+        Self { path }
+    }
+}