Enhance XMP/IPTC canonicalization and mappings

Add broad support for canonicalizing legacy XMP/IPTC children and promoting mixed/legacy namespaces into portable forms. xmp_dump.cc: map Iptc4xmpExt children (Identifier -> xmp, GPS* -> exif), promote xmpMM Pantry.format to dc:format, and refine dc/xmpRights shape detection (LangAlt/Indexed/Structured). Introduce skip-entry checks and multiple new helper functions to resolve structured/indexed/ nested children to portable names and to detect existing explicit properties (prevent duplicate promotions). Add IPTC dataset -> portable structured mappings and claim/emit logic so IPTC datasets like LocationCreated map into structured portable properties. tests: update and add unit tests verifying promotions, canonicalizations, and that legacy entries are not duplicated when canonical replacements exist.

Author: ssh4net