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

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

solitaire_gpgs/src/android.rs

@@ -0,0 +1,12 @@
+// TODO (Phase: Android) — implement JNI bindings here.
+//
+// Steps:
+// 1. Add `jni` dependency under [target.'cfg(target_os = "android")'.dependencies]
+// 2. Implement GpgsClient using cargo-mobile2 JNI bridge
+// 3. pull():  call PlayGames.getSnapshotsClient().open("solitaire_quest_sync")
+//             -> deserialize JSON blob into SyncPayload
+// 4. push():  serialize SyncPayload to JSON -> write to Saved Game slot
+// 5. mirror_achievement(id): call PlayGames.getAchievementsClient().unlock(map_id(id))
+// 6. Maintain a static ID mapping: our &str IDs -> GPGS achievement IDs (from Play Console)
+// 7. On GameWonEvent, submit score to GPGS leaderboard
+// 8. Add Google Sign-In button to Settings screen (Android build only, #[cfg] gated)

solitaire_gpgs/src/lib.rs

@@ -0,0 +1,11 @@
+#[cfg(target_os = "android")]
+mod android;
+
+#[cfg(not(target_os = "android"))]
+mod stub;
+
+#[cfg(not(target_os = "android"))]
+pub use stub::GpgsClient;
+
+#[cfg(target_os = "android")]
+pub use android::GpgsClient;

solitaire_gpgs/src/stub.rs

@@ -0,0 +1,38 @@
+use async_trait::async_trait;
+use solitaire_data::{SyncError, SyncProvider};
+use solitaire_sync::{SyncPayload, SyncResponse};
+
+/// Desktop/iOS stub — always returns UnsupportedPlatform.
+/// Real implementation lives in android.rs (Phase: Android).
+pub struct GpgsClient;
+
+impl GpgsClient {
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+impl Default for GpgsClient {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[async_trait]
+impl SyncProvider for GpgsClient {
+    async fn pull(&self) -> Result<SyncPayload, SyncError> {
+        Err(SyncError::UnsupportedPlatform)
+    }
+
+    async fn push(&self, _payload: &SyncPayload) -> Result<SyncResponse, SyncError> {
+        Err(SyncError::UnsupportedPlatform)
+    }
+
+    fn backend_name(&self) -> &'static str {
+        "Google Play Games (unavailable on this platform)"
+    }
+
+    fn is_authenticated(&self) -> bool {
+        false
+    }
+}