feat: CalDAV foundation — HTTP methods and XML parser

Add PROPFIND and REPORT to HttpMethod enum for CalDAV protocol support.
Implement minimal XML parser (caldav_xml.hpp) for CalDAV response parsing:
namespaced tags, attributes, CDATA, entity references, self-closing elements.
Bounded by max depth and size to prevent allocation bombs.

include/nocal/sync/caldav_xml.hpp

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+namespace nocal::sync {
+
+// A parsed XML element.
+struct XmlElement {
+    std::string tag;
+    std::vector<std::pair<std::string, std::string>> attributes;
+    std::string text;
+    std::string cdata;
+    std::vector<XmlElement> children;
+};
+
+// Parse an XML document string into a tree of XmlElement.
+// Returns the root element. Throws std::runtime_error on parse failure.
+// Bounded by max_size to prevent allocation bombs.
+[[nodiscard]] XmlElement parse_xml(
+    std::string_view document, std::size_t max_size = 4 * 1024 * 1024);
+
+// Find all children with the given tag name.
+[[nodiscard]] std::vector<const XmlElement*> find_children(
+    const XmlElement& parent, std::string_view tag);
+
+// Get first child text or empty string.
+[[nodiscard]] std::string_view get_child_text(
+    const XmlElement& parent, std::string_view tag);
+
+// Get attribute value or empty string.
+[[nodiscard]] std::string_view get_attribute(
+    const XmlElement& elem, std::string_view name);
+
+} // namespace nocal::sync

include/nocal/sync/http.hpp

@@ -5,7 +5,7 @@
 
 namespace nocal::sync {
 
-enum class HttpMethod { get, post, put, patch, delete_ };
+enum class HttpMethod { get, post, put, patch, delete_, propfind, report };
 
 struct HttpHeader {
     std::string name;