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

schema/dialect/sqlite3/sqlite3.go

@@ -13,7 +13,7 @@ import (
 )
 
 var (
-	sqlite3DefaultModifiers = []string{"VirtualAs", "StoredAs", "Nullable", "Default", "Increment"}
+	sqlite3DefaultModifiers = []string{"VirtualAs", "StoredAs", "Hidden", "Nullable", "Default", "Increment"}
 	sqlite3Serials          = []string{"bigInteger", "integer", "mediumInteger", "smallInteger", "tinyInteger"}
 )
 
@@ -178,8 +178,13 @@ func (this Sqlite3) GetColumnModifier(modifier string, bp *schema.Blueprint, col
 		if v := column.GetStoredAs(); v != "" {
 			return this.GenerateSQL(" GENERATED ALWAYS AS (?) STORED", db.L(v))
 		}
+	case "Hidden":
+		if column.IsHidden() {
+			return "HIDDEN"
+		}
 	case "Nullable":
-		if column.GetVirtualAs() == "" && column.GetStoredAs() == "" {
+		// HIDDEN 列和生成列不加 NULL/NOT NULL
+		if column.GetVirtualAs() == "" && column.GetStoredAs() == "" && !column.IsHidden() {
 			if column.IsNullable() {
 				return "NULL"
 			}
@@ -267,10 +272,13 @@ func (this Sqlite3) GenerateSQL(sql string, args ...any) string {
 }
 
 func init() {
-	schema.RegisterDialect("sqlite3", func(db *db.Database) schema.Schema {
+	sc := func(db *db.Database) schema.Schema {
 		return &Sqlite3{
 			db:  db,
 			esg: sqlgen.NewExpressionSQLGenerator("sqlite3", sqlite3.DialectOptions()),
 		}
-	})
+	}
+
+	schema.RegisterDialect("sqlite3", sc)
+	schema.RegisterDialect("vtable", sc)
 }