Phase 2: DataFusion query engine over Parquet

- queryd: SessionContext with custom URL scheme to avoid path doubling with LocalFileSystem
- queryd: ListingTable registration from catalog ObjectRefs with schema inference
- queryd: POST /query/sql returns JSON {columns, rows, row_count}
- queryd→catalogd wiring: reads all datasets, registers as named tables
- gateway: wires QueryEngine with shared store + registry
- e2e verified: SELECT *, WHERE/ORDER BY, COUNT/AVG all correct

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/gateway/src/main.rs

@@ -19,11 +19,14 @@ async fn main() {
         tracing::warn!("catalog rebuild failed (empty store?): {e}");
     }
 
+    // Query engine — DataFusion over catalog-registered Parquet
+    let engine = queryd::context::QueryEngine::new(registry.clone(), store.clone());
+
     let app = Router::new()
         .route("/health", get(health))
         .nest("/storage", storaged::service::router(store))
         .nest("/catalog", catalogd::service::router(registry))
-        .nest("/query", queryd::service::router())
+        .nest("/query", queryd::service::router(engine))
         .nest("/ai", aibridge::service::router())
         .layer(TraceLayer::new_for_http());