feat(sql): Window Functions + MERGE statement + REST bridge plan

- Add Window Functions: ROW_NUMBER, RANK, DENSE_RANK, LEAD, LAG,
  FIRST_VALUE, LAST_VALUE, NTILE with PARTITION BY, ORDER BY,
  ROWS/RANGE frame specifications
- Add MERGE/UPSERT statement with WHEN MATCHED UPDATE and
  WHEN NOT MATCHED INSERT
- Add SQL/PGQ Property Graph long-term plan in PLAN_SQL_ADVANCED.md
- Add 7 new tests for Window Functions and MERGE
- Update baraql.md documentation

docs/en/clients.md

@@ -45,6 +45,18 @@ await client.commit();
 await client.close();
 ```
 
+### Concurrent Queries
+
+The JavaScript client automatically serializes concurrent requests over a single TCP connection via an internal request queue. You can safely fire multiple parallel operations — their binary frames will not interleave on the wire:
+
+```typescript
+const [users, orders, stats] = await Promise.all([
+  client.query('SELECT * FROM users'),
+  client.query('SELECT * FROM orders'),
+  client.query('SELECT count(*) FROM visits')
+]);
+```
+
 ### WebSocket Streaming
 
 ```typescript