test(fifa17): keep economy harness serialized; document multi-conn blocker

WAL-establish-once + busy_timeout (core 75b1830) reduced but did not eliminate a
brand-new-DB multi-connection warm-up 'database error'; the E2E+restart harness
stays on a single serialized connection for determinism, and the residual
multi-connection concurrency issue is documented as the remaining Part-T blocker.

openfut-utas-host/tests/economy_integration.rs

@@ -21,11 +21,12 @@ use serde_json::{json, Value};
 /// Boot a Core instance against `db_url`, serving on an ephemeral port. Returns
 /// the serve task handle and its base URL.
 async fn start_core(db_url: &str) -> (tokio::task::JoinHandle<()>, String) {
-    // One pooled connection: the harness drives Core sequentially (blocking
-    // client), so a single connection serializes access deterministically and
-    // avoids a WAL-mode-establishment race across connections warming up on a
-    // brand-new DB file. (Steady-state multi-connection concurrency is a
-    // separate, deferred requirement.)
+    // Serialize Core access with a single pooled connection so this E2E+restart
+    // harness is deterministic. NOTE: even with the WAL-establish-once +
+    // busy_timeout fixes (core 75b1830), a multi-connection pool on a brand-new
+    // DB still intermittently surfaces a write "database error" under warm-up —
+    // a deeper sqlx/SQLite pool concurrency issue that is a documented remaining
+    // blocker for the Part-T concurrency proof, not exercised by this harness.
     let pool = openfut_core::db::init_pool(db_url, 1)
         .await
         .expect("core pool");