Gracefully deprecate halfLimits.h

halfLimits.h is no longer needed but the deprecation warning is confusing for code that must support earlier library versions. There should be a way to suppress the warning.

Start by inspecting halfLimits.h and the existing deprecation warning to understand how the header is currently marked obsolete and how compatibility with earlier library versions is handled. Determine an explicit opt-out mechanism for projects that still include the header, and consider how its behavior should be verified before documenting the completed interface.

Written by the indexing model from the issue text.