feat: 新增 SQLite3 虚拟表框架及方言扩展

- 新增 dialect/sqlite3/vtab 包，提供干净的虚拟表接口（Module/Table/Cursor）
- 适配层自动将 BestIndex 约束与 Filter 值绑定（ConstraintInfo.Value），无需手动编解码 IdxStr
- 支持 OpLIMIT/OpOFFSET 约束下推
- 新增 SupportsDistinct 方言选项，控制 SELECT 级和表达式级 DISTINCT 生成
- sqlite3 方言注册 IF() 函数支持

sqlgen/expression_sql_generator.go

@@ -559,6 +559,15 @@ func (esg *expressionSQLGenerator) literalExpressionSQL(b sb.SQLBuilder, literal
 //
 //	COUNT(I("a")) -> COUNT("a")
 func (esg *expressionSQLGenerator) sqlFunctionExpressionSQL(b sb.SQLBuilder, sqlFunc exp.SQLFunctionExpression) {
+	if sqlFunc.Name() == "DISTINCT" && !esg.dialectOptions.SupportsDistinct {
+		for i, arg := range sqlFunc.Args() {
+			if i > 0 {
+				b.WriteRunes(esg.dialectOptions.CommaRune, esg.dialectOptions.SpaceRune)
+			}
+			esg.Generate(b, arg)
+		}
+		return
+	}
 	b.WriteStrings(sqlFunc.Name())
 	esg.Generate(b, sqlFunc.Args())
 }

sqlgen/sql_dialect_options.go

@@ -34,6 +34,8 @@ type (
 		SupportsWithCTERecursive bool
 		// Set to true if multiple tables are supported in UPDATE statement. (DEFAULT=true)
 		SupportsMultipleUpdateTables bool
+		// Set to true if DISTINCT is supported (DEFAULT=true)
+		SupportsDistinct bool
 		// Set to true if DISTINCT ON is supported (DEFAULT=true)
 		SupportsDistinctOn bool
 		// Set to true if LATERAL queries are supported (DEFAULT=true)
@@ -420,6 +422,7 @@ func DefaultDialectOptions() *SQLDialectOptions {
 		SupportsConflictTarget:      true,
 		SupportsWithCTE:             true,
 		SupportsWithCTERecursive:    true,
+		SupportsDistinct:            true,
 		SupportsDistinctOn:          true,
 		WrapCompoundsInParens:       true,
 		SupportsWindowFunction:      true,