Redesign SharedSingleton<T>: explicit Get/Install/Reset and thread safety

[gagelarsen]

Follow-up from issue #116 (review finding #9).

xmscore/misc/Singleton.h:81-121 — SharedSingleton<T>::Instance(Ptr a_new = Ptr(), bool a_delete = false) has the same conflated get/install/destroy shape as Singleton<T> (#116-followup-singleton), plus:

• A second installer call silently clobbers the first. The precondition assert is commented out.
• No documented thread-safety expectations; concurrent first-time installs race.

Proposed change

• Replace with explicit methods:
  • Get() — returns the current shared_ptr.
  • Install(Ptr) — install a replacement; either fail loudly on a second install or document the policy.
  • Reset() — tear down.
• Either guard install with std::call_once / a std::mutex, or document explicitly that callers are responsible for serializing installs.
• Document interaction with in-flight users of the previous instance.

Why follow-up, not part of #117

The new \brief in #117 names this type as the canonical CRTP base for xmscore singletons. The actual fix needs its own type-design review.

Acceptance

• Get() / Install() / Reset() exist; combined Instance(Ptr, bool) marked deprecated
• Second Install() either errors loudly or is documented as last-write-wins
• Thread-safety policy documented and (where claimed) enforced
• Foundation singletons (XmLog, ProgressListener) audited against the new shape

Refs: issue #116, PR #117.