[Alexander-Mitrofanov]

The full issue #244 C++23 assessment existed only as an untracked document. This PR preserves it as a historical, revision-pinned audit, together with the standalone C++23/C++17 library probes and the explicitly set-aside C++17 compatibility patch and notes. The files are registered in the source distribution and ChangeLog.

The probes exercise string library APIs and unordered_map::try_emplace. The archived patch demonstrates the older proposed replacements for C++23 strings/stream views and an older compiler CI matrix. It is historical review material: the build does not apply it, and it has not been rebased or validated against current master. Current compiler requirements remain as implemented.

Why it was not committed before: the saved consensus and public issue #244 comment record a decision to retain C++23 with no source/configuration change. The shorter conclusion was published on the issue; the longer audit remained untracked in audit/244-cxx23-rationale, and the rejected compatibility experiment remained under issue-244-validation/set-aside-cxx17.*. This PR preserves that local investigation before the workspace is removed, without presenting the abandoned proposal as a supported implementation.

The assessment explicitly names its September 21 source revision. Later changes such as the mdspan migration are outside its scope; historic line references and performance conclusions are not claims about current master.

Validation: both saved probes compile and run with GCC 14.4 in their corresponding C++23/C++17 modes and -pedantic-errors; the archived patch parses as a 12-file unified diff; make dist preserves all six archived documentation/probe files and local documentation links were checked; staged whitespace checks pass. No application code changes, so no full application rebuild is required.

Related: #244, #234, #248.