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

dist/metona-sqlark.esm.js

@@ -3997,6 +3997,12 @@ class QueryExecutor {
         return this.engine.delete(plan.table, plan);
     }
     async executeCreateTable(stmt) {
+        // 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 = {};
         for (const col of stmt.columns)
             columns[col.name] = astColumnToColumnDef(col);
@@ -4759,6 +4765,14 @@ class Parser {
     parseCreateTable() {
         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);
         const columns = [];
@@ -4768,7 +4782,7 @@ class Parser {
             columns.push(this.parseColumnDef());
         } while (this.curTokenIs(TokenType.COMMA));
         this.expect(TokenType.RPAREN);
-        return { type: 'CREATE_TABLE', name: tableName, columns };
+        return { type: 'CREATE_TABLE', name: tableName, columns, ifNotExists: ifNotExists || undefined };
     }
     parseColumnDef() {
         const name = this.expectIdentifier('column name');