Write doxygen XML outside the published tree, re-enable doxystat

[meshinspector-agent]

XML_OUTPUT was a relative path, so Doxygen resolved it against OUTPUT_DIRECTORY — the published directory — and the deploy step's git add -A committed the XML. Nothing reads it except the doxystat metrics, which are themselves disabled.

Changed

• update_doc.sh: XML_OUTPUT → an absolute, git-ignored doxystat-xml/ at the repo root (both in generate_documentation_simple() and the currently-unused generate_documentation()).
• Re-enabled show_statistics(), reading that dir and tee-ing to doxystat-metrics.txt so CI can upload it as an artifact.
• .gitignore: added doxystat-xml/, doxystat-metrics.txt, /stats.txt; widened MeshLib/xml/ → MeshLib/xml*/ (same under dev/) so a regressed path can't leak in.

Surfaced by re-enabling doxystat — dead code until now, so neither was ever hit:

• doxystat/{index,compound}.py (generateDS output) need six, and their namespace handling only works with lxml; the xml.etree fallback they import raises AttributeError: … has no attribute 'nsmap'. Hence the new scripts/doxystat/requirements.txt plus a guarded install.
• metrics.py writes stats.txt into the cwd — i.e. the repo root, a new published junk file. It now runs from inside the XML dir, so each module's stats.txt lands beside its XML.

Metrics can never fail the docs run: missing XML or uninstallable dependencies warn and return 0.

Deleting the already-committed xml_* and dev/test/ is a separate change that must land after this one, or the next docs run re-commits the XML.

Verification

No doxygen 1.14 or bindings artifacts available locally, so I drove the real update_doc.sh in a sandbox against a stub doxygen implementing the documented rule ("if a relative path is entered the value of OUTPUT_DIRECTORY will be put in front of it").

• Before (master's script + .gitignore): git add -A stages MeshLib/dev/xml_Cpp/* and MeshLib/dev/xml_Py/* — bug reproduced.
• After: doxygen is invoked with XML_OUTPUT=[/…/doxystat-xml/xml_Cpp]; the published tree has no xml* directory; git add -A stages only MeshLib/dev/html/**.
• git check-ignore -v confirms coverage of doxystat-xml/, doxystat-metrics.txt, MeshLib/xml_*/, MeshLib/dev/xml_*/ and the old MeshLib/xml/.
• metrics.py really parses the out-of-tree XML — doxystat-metrics.txt gets a Classes: 1 (1 documented) table per module.
• Guards: XML dir missing → [WARN] no doxygen XML for Py, skipping its metrics, rc=0; dependencies unavailable → [WARN] could not install doxystat dependencies, skipping metrics, rc=0.
• bash -n clean.

Not tested: a real doxygen 1.14 run. HTML generation is untouched, but the first CI docs run is the real check — worth confirming there that doxystat-metrics.txt is non-empty and /dev/ pages still render.

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