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

engine/engine_config.go

@@ -88,7 +88,7 @@ func (c *DBConfig) ToDSN() string {
 		return c.toMySQLDSN()
 	case "pgsql":
 		return c.toPostgreSQLDSN()
-	case "sqlite3":
+	case "sqlite3", "vtable":
 		return c.toSQLiteDSN()
 	case "sqlserver":
 		return c.toSQLServerDSN()
@@ -328,6 +328,7 @@ func NewDBConfig(driver string, options ...Option) DBConfig {
 		"sqlite3":   true,
 		"sqlserver": true,
 		"duckdb":    true,
+		"vtable":    true,
 	}
 	if !validDrivers[driver] {
 		panic(fmt.Sprintf("Unsupported driver: %s", driver))
@@ -363,7 +364,7 @@ func NewDBConfig(driver string, options ...Option) DBConfig {
 		if config.Password == "" {
 			panic(fmt.Sprintf("Password is required for %s driver", config.Driver))
 		}
-	case "sqlite3":
+	case "sqlite3", "vtable":
 		if config.SQLite.File == "" {
 			panic("File is required for sqlite3 driver")
 		}
@@ -429,7 +430,7 @@ func WithWriteHosts(hosts []string) Option {
 	}
 }
 
-// MySQL 专用选项
+// WithMySQLCollation MySQL 专用选项
 func WithMySQLCollation(collation string) Option {
 	return func(c *DBConfig) {
 		if c.Driver != "mysql" {
@@ -461,7 +462,7 @@ func WithPgSslmode(sslmode string) Option {
 // SQLite 专用选项
 func WithSQLiteFile(file string) Option {
 	return func(c *DBConfig) {
-		if c.Driver != "sqlite3" {
+		if c.Driver != "sqlite3" && c.Driver != "vtable" {
 			panic("WithSQLiteFile is only valid for sqlite3 driver")
 		}
 		c.SQLite.File = file
@@ -470,7 +471,7 @@ func WithSQLiteFile(file string) Option {
 
 func WithSQLiteJournal(journal string) Option {
 	return func(c *DBConfig) {
-		if c.Driver != "sqlite3" {
+		if c.Driver != "sqlite3" && c.Driver != "vtable" {
 			panic("WithSQLiteJournal is only valid for sqlite3 driver")
 		}
 		c.SQLite.Journal = journal