feat: 新增 DuckDB 方言支持及 SQLite3 自动注册 IF 函数

- 新增 dialect/duckdb 方言，支持 DuckDB SQL 语法
- 新增 schema/dialect/duckdb DDL 操作支持
- dialect/sqlite3: 注册 sqlite3_with_if 驱动，连接时自动注册 IF() 函数
- engine: MakeConnection 对 sqlite3 自动使用带 IF 支持的驱动
- engine: 新增 DBConfig DuckDB 配置项及相关 Option 函数
- 统一各方言测试引用路径

dialect/duckdb/duckdb.go

@@ -0,0 +1,38 @@
+package duckdb
+
+import (
+	_ "github.com/marcboeker/go-duckdb"
+
+	"git.fsdpf.net/go/db"
+)
+
+func DialectOptions() *db.SQLDialectOptions {
+	do := db.DefaultDialectOptions()
+
+	// DuckDB uses PostgreSQL-style placeholders ($1, $2, etc.)
+	do.PlaceHolderFragment = []byte("$")
+	do.IncludePlaceholderNum = true
+
+	// DuckDB supports most PostgreSQL features
+	do.SupportsReturn = true
+	do.SupportsOrderByOnUpdate = false
+	do.SupportsLimitOnUpdate = false
+	do.SupportsOrderByOnDelete = false
+	do.SupportsLimitOnDelete = false
+	do.SupportsConflictUpdateWhere = true
+	do.SupportsInsertIgnoreSyntax = false
+	do.SupportsConflictTarget = true
+	do.SupportsWithCTE = true
+	do.SupportsWithCTERecursive = true
+	do.SupportsDistinctOn = true
+	do.SupportsWindowFunction = true
+
+	// Use double quotes for identifiers (PostgreSQL-style)
+	do.QuoteRune = '"'
+
+	return do
+}
+
+func init() {
+	db.RegisterDialect("duckdb", DialectOptions())
+}