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/sql/parser.ts

@@ -327,6 +327,16 @@ export class Parser {
   private parseCreateTable(): CreateTableStatement {
     this.expect(TokenType.CREATE);
     this.expect(TokenType.TABLE);
+
+    // IF NOT EXISTS（可选）
+    let ifNotExists = false;
+    if (this.curTokenIs(TokenType.IF)) {
+      this.nextToken();
+      this.expect(TokenType.NOT);
+      this.expect(TokenType.EXISTS);
+      ifNotExists = true;
+    }
+
     const tableName = this.expectIdentifier('table name');
     this.expect(TokenType.LPAREN);
 
@@ -338,7 +348,7 @@ export class Parser {
 
     this.expect(TokenType.RPAREN);
 
-    return { type: 'CREATE_TABLE', name: tableName, columns };
+    return { type: 'CREATE_TABLE', name: tableName, columns, ifNotExists: ifNotExists || undefined };
   }
 
   private parseColumnDef(): ASTColumnDef {