The runner's hand-written JSON parser and writer, and its XML escaping,
are replaced by two established MIT libraries, vendored in third_party/:
yyjson 0.13.0 and pugixml 1.16. JsonValue and the JUnit model stay as they
are.

Both are compiled into Catch3's own translation units, so a project that
links its own yyjson or pugixml is not affected:
- src/Progmasoft/Runner/Yyjson.inl includes yyjson.c into Json.cpp with
  yyjson_api defined as static, so no yyjson symbol leaves that file.
- src/Progmasoft/PugiXml.inl includes pugixml header-only into
  XmlWriter.cpp with its namespace renamed to Progmasoft_Catch3_Pugi.
- Both are read as system headers, outside the warnings Catch3 builds
  with; they are .inl files, which the header install rules leave out.

JSON keeps its behaviour: strings are bytes and come back as written
(invalid UTF-8 is allowed both ways), non-finite numbers are written as
null, documents nested deeper than 256 levels are refused while they are
converted, and whole numbers are written as integers. Fractions are now
written as the shortest text that reads back as the same double.

JUnit XML stays equivalent; pugixml writes ' and > in attributes as they
are, U+FFFD as the character itself and empty elements as <x />. The
tests follow.

third_party/{yyjson,pugixml} each have BUILD.bazel, CMakeLists.txt,
meson.build, xmake.lua and premake5.lua, which only provide the include
directory to the library; CMake links them through BUILD_INTERFACE.

tools/scripts/updateThirdParty.py pins the version and sha256 of each
archive; --check, now a step of the header and license workflow, fails
when a vendored file differs from its archive, and --update writes them.
Renovate proposes new versions through a regex manager on that table.

The amalgamation now has a third file, catch_amalgamated_third_party.hpp,
with both wrappers and the libraries inlined; catch_amalgamated.cpp
includes it once. The documents and the release notes say so.