Add archive helpers and harden task runtime

site/doc/pages/xml_encode.txt

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+:sig
+String xml_encode(DTree t)
+String xml_encode(DTree t, String root_name)
+
+:params
+t : tree to serialize as XML
+root_name : optional element name to use when `t` is not already an element-shaped tree, defaults to `root`
+return value : XML string
+
+:see
+>markup
+xml_decode
+json_encode
+0_DTree
+html_escape
+
+:content
+Serializes a `DTree` 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()`.
+
+Try the live example in the [XML demo](../demo/xml.uce).
+
+The native element shape is:
+
+```text
+node["name"] = "book"
+node["attrs"]["id"] = "b1"
+node["text"] = "optional text"
+node["children"] = list of child element nodes
+```
+
+Example:
+
+```uce
+DTree book;
+book["name"] = "book";
+book["attrs"]["id"] = "b1";
+
+DTree title;
+title["name"] = "title";
+title["text"] = "UCE & XML";
+book["children"].push(title);
+
+String xml = xml_encode(book);
+```
+
+The result is:
+
+```xml
+<book id="b1"><title>UCE &amp; XML</title></book>
+```
+
+For simple map/list/scalar trees, `xml_encode()` creates ordinary child elements:
+
+```uce
+DTree payload;
+payload["title"] = "Hello";
+payload["count"] = "3";
+
+String xml = xml_encode(payload, "payload");
+```
+
+Result:
+
+```xml
+<payload><count>3</count><title>Hello</title></payload>
+```
+
+Notes:
+
+- Element and attribute names are normalized to XML-name-safe strings when needed.
+- 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.