feat(workspace): initialize all seven crates with stubs and blank Bevy window

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_sync/src/lib.rs

@@ -0,0 +1,17 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+/// Payload sent from client to server (and returned after server merge).
+/// Full fields are added in Phase 8 (Sync System).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SyncPayload {
+    pub user_id: Uuid,
+    pub last_modified: DateTime<Utc>,
+}
+
+/// Response returned by the sync server after merging.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SyncResponse {
+    pub server_time: DateTime<Utc>,
+}