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

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

solitaire_data/src/lib.rs

@@ -0,0 +1,30 @@
+use async_trait::async_trait;
+use solitaire_sync::{SyncPayload, SyncResponse};
+use thiserror::Error;
+
+/// All errors that can arise during sync operations.
+#[derive(Debug, Error)]
+pub enum SyncError {
+    #[error("unsupported platform for this sync backend")]
+    UnsupportedPlatform,
+    #[error("network error: {0}")]
+    Network(String),
+    #[error("authentication error: {0}")]
+    Auth(String),
+    #[error("serialization error: {0}")]
+    Serialization(String),
+}
+
+/// Every sync backend implements this trait. The SyncPlugin only calls these
+/// methods — it never matches on a backend enum variant.
+#[async_trait]
+pub trait SyncProvider: Send + Sync {
+    async fn pull(&self) -> Result<SyncPayload, SyncError>;
+    async fn push(&self, payload: &SyncPayload) -> Result<SyncResponse, SyncError>;
+    fn backend_name(&self) -> &'static str;
+    fn is_authenticated(&self) -> bool;
+    /// Mirror an achievement unlock to this backend (no-op for most backends).
+    async fn mirror_achievement(&self, _id: &str) -> Result<(), SyncError> {
+        Ok(())
+    }
+}