Collapse STRstd<T> four-overload set into one specialized template

[gagelarsen]

Follow-up from issue #116 (review MINOR finding on xmscore/misc/StringUtil.h:983-998).

The new \brief paragraph for STRstd<T> (added in PR #117's predecessor docs commits) explains the overload-resolution traps the four explicit overloads exist to dodge. The reviewer's note: this is a signal that the four-overload set could likely be collapsed into one specialized template, with the special cases handled via if constexpr (C++17) or tag dispatch.

Proposed change

• Replace the four free STRstd overloads with a single primary template plus the specializations actually needed. Use if constexpr on std::is_floating_point_v<T> / std::is_integral_v<T> where the current overloads diverge.
• Verify against existing call sites (downstream xms libraries call this through STRstd directly).
• Update the \brief once the overload-trap discussion is no longer needed.

Why follow-up

The reviewer explicitly tagged this as FOR FOLLOW-UP ("signal that the four-overload set could become one specialized template"). It's a real refactor with downstream impact, not a doc fix.

Acceptance

• One template (with specializations as needed) replaces the four overloads
• All existing call sites in xmscore continue to compile
• No behavior change in CxxTest coverage of StringUtil
• \brief updated to drop the overload-trap explanation

Refs: issue #116, PR #117, xmscore/misc/StringUtil.h:983-998.