feat: SQL 语法支持 CREATE TABLE IF NOT EXISTS

- Parser 支持 CREATE TABLE IF NOT EXISTS 语法
- AST CreateTableStatement 新增 ifNotExists 字段
- Executor 表已存在时静默跳过
- 修复 demo.html Aria 预设报错

src/query/executor.ts

@@ -228,6 +228,11 @@ export class QueryExecutor {
   }
 
   private async executeCreateTable(stmt: CreateTableStatement): Promise<void> {
+    // IF NOT EXISTS: 表已存在时静默返回
+    if (stmt.ifNotExists) {
+      const exists = await this.engine.hasTable(stmt.name);
+      if (exists) return;
+    }
     const columns: Record<string, any> = {};
     for (const col of stmt.columns) columns[col.name] = astColumnToColumnDef(col);
     return this.engine.createTable(createSchema(stmt.name, columns));