Build the Doxygen reference in CI and fail on warnings

The Doxyfile had never run in CI, and its warnings went to a log file.

- It reads the headers only (*.hpp): Doxygen cannot match the explicit
  specializations of StringMaker and the PImpl constructors defined in the
  sources to their declarations, which gave ten of its twelve warnings.
- src/Progmasoft/Catch3/Internal/Preprocessor.hpp is left out: Doxygen reads
  priority_tag<N> deriving from priority_tag<N - 1> as a recursive class.
- WARN_AS_ERROR = FAIL_ON_WARNINGS, warnings on standard error.
- DOT_GRAPH_MAX_NODES = 200, so the include graph of All.hpp (103 nodes)
  is drawn; DOT_MULTI_TARGETS, which Doxygen 1.18 no longer knows, is gone.
- .github/workflows/doxygen.yml installs Doxygen and Graphviz, builds the
  reference and keeps the HTML.

Claude-Session: https://claude.ai/code/session_01Ji7xZMYjc4zankoc7hGBE1

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