Gate engine template instantiations by PGen compatibility

[haykh]

Motivation

• The build was instantiating Engine templates for every entry in NTT_FOREACH_SPECIALIZATION, which triggered template constraint failures for engine/metric/dimension combinations that are incompatible with the selected problem generator (PGen).
• The intent is to avoid hard instantiation of invalid combinations at compile time and only force instantiation for combinations allowed by the problem generator compatibility traits.

Description

• Replaced direct explicit instantiations with compatibility-gated instantiator helpers in src/engines/engine_init.cpp, src/engines/engine_run.cpp, and src/engines/engine_printer.cpp by adding EngineInitInstantiator, EngineRunInstantiator, and EnginePrinterInstantiator respectively.
• Each instantiator uses if constexpr (IsCompatibleWithEngine<S, M<D>>) and takes the address of the corresponding member function (for example (void)&Engine<S, M<D>>::init) so the compiler only instantiates members for compatible combinations instead of all specializations.
• The NTT_FOREACH_SPECIALIZATION expansion now emits unique static instantiator variables using __COUNTER__ and small NTT_CONCAT macros to avoid global template instantiations for incompatible combos, and related cleanup of macros was added.
• This change gates init, run, and print_report instantiations by PGen compatibility, preventing the earlier compile-time constraint failures.

Testing

• No automated tests or builds were run for this change.

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Codex Task