refactor(xml, png): give the xml and png helpers their own internal packages

[andiwand]

🤖 Generated with Claude Code

util/xml_util moves into the existing xml package, next to XmlFile, and util/png_util becomes a package of its own. The namespaces follow: util::xml → xml, util::png → png.

util is for helpers that belong to no engine, and neither of these does. Every xml-reading and xml-writing engine goes through the parse and the escaping; the png writer is the encoder pdf image extraction and svm bitmaps hand their pixels to. The xml half also lands where xml:: already resolved for open_strategy, svg_file and odf_flat_file — those files were spelling the namespace before the helpers arrived in it.

from	to
src/odr/internal/util/xml_util.{hpp,cpp}	src/odr/internal/xml/xml_util.{hpp,cpp}
src/odr/internal/util/png_util.{hpp,cpp}	src/odr/internal/png/png_util.{hpp,cpp}
test/src/internal/util/xml_util_test.cpp	test/src/internal/xml/xml_util_test.cpp
test/src/internal/util/png_util_test.cpp	test/src/internal/png/png_util_test.cpp

Contents are unchanged apart from the namespace; the rest of the diff is call sites (xml::parse, xml::escape_attribute, png::write), the two CMake source lists, and re-sorted includes.

Docs: the root AGENTS.md directory map (the util/ row, a new png/ row, the xml/ row, and the pugixml-layout note that cites xml_util.cpp), xml/AGENTS.md — which gains a short section saying the package is now shared plumbing and a format — svg/AGENTS.md, which cited a util::xml::escape that had not gone by that name for a while, and docs/design/editing.md.

No CHANGELOG.md entry: refactoring is in the generated per-PR list already.

Testing

odr and odr_test build clean. 63 tests across the xml, png, svg, ooxml, odf and pdf-image suites pass. The data-driven suites did not run — test/data is not fetched in this checkout — so CI is the first place the full suite sees this.