fix(workspace): add derives/docs per code review, remove unused thiserror from solitaire_sync

Cargo.lock

@@ -6815,7 +6815,6 @@ dependencies = [
  "chrono",
  "serde",
  "serde_json",
- "thiserror 1.0.69",
  "uuid",
 ]
 

solitaire_data/src/lib.rs

@@ -7,6 +7,8 @@ use thiserror::Error;
 pub enum SyncError {
     #[error("unsupported platform for this sync backend")]
     UnsupportedPlatform,
+    // TODO: Replace String with concrete source error types (e.g. reqwest::Error,
+    // serde_json::Error) when real implementations are added in Phase 8.
     #[error("network error: {0}")]
     Network(String),
     #[error("authentication error: {0}")]
@@ -19,9 +21,13 @@ pub enum SyncError {
 /// methods — it never matches on a backend enum variant.
 #[async_trait]
 pub trait SyncProvider: Send + Sync {
+    /// Fetch the remote sync payload. Returns the latest server state for merging.
     async fn pull(&self) -> Result<SyncPayload, SyncError>;
+    /// Push the local payload to the backend. Returns the merged server response.
     async fn push(&self, payload: &SyncPayload) -> Result<SyncResponse, SyncError>;
+    /// Human-readable name of this backend, used in settings UI and logs.
     fn backend_name(&self) -> &'static str;
+    /// Returns true if the user is currently authenticated with this backend.
     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> {

solitaire_gpgs/src/stub.rs

@@ -2,11 +2,14 @@ use async_trait::async_trait;
 use solitaire_data::{SyncError, SyncProvider};
 use solitaire_sync::{SyncPayload, SyncResponse};
 
-/// Desktop/iOS stub — always returns UnsupportedPlatform.
+/// Google Play Games Services sync client — desktop/iOS stub.
-/// Real implementation lives in android.rs (Phase: Android).
+///
+/// Always returns [`SyncError::UnsupportedPlatform`]. The real JNI implementation
+/// lives in `android.rs` and is compiled only on Android (Phase: Android).
 pub struct GpgsClient;
 
 impl GpgsClient {
+    /// Creates a new `GpgsClient` stub. No-op on non-Android platforms.
     pub fn new() -> Self {
         Self
     }

solitaire_sync/Cargo.toml

@@ -8,4 +8,3 @@ serde      = { workspace = true }
 serde_json = { workspace = true }
 uuid       = { workspace = true }
 chrono     = { workspace = true }
-thiserror  = { workspace = true }

solitaire_sync/src/lib.rs

@@ -4,14 +4,14 @@ 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)]
+#[derive(Debug, Clone, PartialEq, Eq, 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)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct SyncResponse {
     pub server_time: DateTime<Utc>,
 }