refactor: rename DTree to DValue

site/doc/pages/xml_encode.txt

@@ -1,6 +1,6 @@
 :sig
-String xml_encode(DTree t)
-String xml_encode(DTree t, String root_name)
+String xml_encode(DValue t)
+String xml_encode(DValue t, String root_name)
 
 :params
 t : tree to serialize as XML
@@ -11,11 +11,11 @@ return value : XML string
 >markup
 xml_decode
 json_encode
-0_DTree
+0_DValue
 html_escape
 
 :content
-Serializes a `DTree` into a simple XML string.
+Serializes a `DValue` into a simple XML string.
 
 `xml_encode()` does not validate against a schema, DTD, or namespace rules. It is a structural converter for application data, similar in spirit to `json_encode()`.
 
@@ -33,11 +33,11 @@ node["children"] = list of child element nodes
 Example:
 
 ```uce
-DTree book;
+DValue book;
 book["name"] = "book";
 book["attrs"]["id"] = "b1";
 
-DTree title;
+DValue title;
 title["name"] = "title";
 title["text"] = "UCE & XML";
 book["children"].push(title);
@@ -54,7 +54,7 @@ The result is:
 For simple map/list/scalar trees, `xml_encode()` creates ordinary child elements:
 
 ```uce
-DTree payload;
+DValue payload;
 payload["title"] = "Hello";
 payload["count"] = "3";
 
@@ -73,4 +73,4 @@ Notes:
 - Text and attribute values are escaped.
 - List values use repeated `<item>` children.
 - Empty elements serialize as self-closing tags.
-- Map child order follows `DTree` map iteration order.
+- Map child order follows `DValue` map iteration order.