Handle indexed/structured lang-alt XMP properties

Extend XMP portable dump support for additional structured shapes: bounded indexed-structured properties and structured child lang-alt properties. Added parsing helpers (parse_indexed_structured_xmp_property_name, parse_structured_lang_alt_xmp_property_name, parse_indexed_structured_lang_alt_xmp_property_name), new Portable* structs to represent structured/lang-alt/indexed items, and claim maps + helpers to track ownership. Implemented emission routines to correctly serialize structured lang-alt children, structured groups, and indexed-structured groups preserving order and language qualifiers. Also register additional namespaces (plus, Iptc4xmpExt) when present in existing XMP and updated grouping/sorting logic and callers in dump_xmp_portable to collect and emit these new property shapes. Tests updated accordingly.

Author: ssh4net

src/include/openmeta/xmp_dump.h

@@ -84,9 +84,11 @@ struct XmpPortableOptions final {
     /// Include \ref MetaKeyKind::XmpProperty entries already present in the store.
     ///
     /// \note Currently simple `property_path` values, indexed `[n]` paths,
-    /// bounded lang-alt paths like `title[@xml:lang=x-default]`, and
-    /// bounded one-level structured paths like
-    /// `CreatorContactInfo/CiEmailWork` are emitted.
+    /// bounded lang-alt paths like `title[@xml:lang=x-default]`, bounded
+    /// one-level structured paths like `CreatorContactInfo/CiEmailWork`,
+    /// bounded indexed-structured paths like `Licensee[1]/LicenseeName`, and
+    /// bounded structured child lang-alt paths like
+    /// `CreatorContactInfo/CiAdrCity[@xml:lang=x-default]` are emitted.
     bool include_existing_xmp = false;
     /// Existing XMP namespace writeback policy for portable output.
     XmpExistingNamespacePolicy existing_namespace_policy