Clean up baseline Doxygen warnings, then enable WARN_AS_ERROR in Docs-CI

[gagelarsen]

Follow-up from issue #116 (review MINOR finding on .github/workflows/XmsCore-CI.yaml).

PR #117 added .github/workflows/Docs-CI.yaml that builds Doxygen and Sphinx and uploads Doxygen/doxy_warn.log as an artifact, but it does not enable WARN_AS_ERROR=YES (or Sphinx -W) yet because there is a baseline of 31 pre-existing Doxygen warnings that would otherwise fail CI on master.

Baseline (as of ab37a329)

```
xmscore/points/pt.h:1632, 1642, 1652 end of comment block while expecting command
(<U> template parameter parsed as HTML )
xmscore/misc/XmLog.cpp:85, 107 return type of member iProcessName / iTimestamp not documented
xmscore/misc/Observer.h:56 parameters of OnProgressStatus not documented
xmscore/misc/Observer.cpp:91 (×2) ProgressStatus has multiple/too-many @PARAM sections
xmscore/points/pt.h:585..714 (~16) documented \return on operators that return void
… plus a handful of MockMesher/MockProgressListener/Impl ctor/dtor missing-param warnings.
```

Plan

1. Fix the <U> template-parameter cases in pt.h (escape with \<U\> or use \tparam instead of inline HTML).
2. Strip \return lines from in-place compound operators that actually return void.
3. Document the missing \param lines (or add [in]/[out] markers where the param doc lives in the header only).
4. Resolve the duplicate \param on Observer::ProgressStatus (header-vs-cpp).
5. Once doxy_warn.log is empty, flip Docs-CI.yaml:
   • Doxygen: pipe WARN_AS_ERROR=YES into the run (or set it in Doxyfile).
   • Sphinx: change make html → make html SPHINXOPTS='-W --keep-going'.

Why follow-up

Cleaning up the baseline and enabling the gate are independent of the issue #116 doc work and have a different review surface (mostly mechanical comment edits across pt.h).

Acceptance

• Doxygen/doxy_warn.log is empty after a clean build
• Docs-CI.yaml runs Doxygen with WARN_AS_ERROR=YES
• Docs-CI.yaml runs Sphinx with -W --keep-going
• CI is green on master

Refs: issue #116, PR #117.