Gate engine explicit instantiations by PGen compatibility

[haykh]

Motivation

• Prevent explicit template instantiations from being generated for engine/metric/dimension combinations that are not supported by the selected problem generator, which caused compile-time template constraint failures when building with some PGENs (e.g. shock).

Description

• Add a new concept IsCompatibleWithPGen<S, M> in src/engines/engine.hpp that checks metric presence and the user::PGen compatibility lists engines, metrics, and dimensions.
• Replace direct explicit instantiations of Engine<S, M<D>>::init, ::run, and ::print_report with small instantiator structs that are only defined when IsCompatibleWithPGen<S, M<D>> holds in src/engines/engine_init.cpp, src/engines/engine_run.cpp, and src/engines/engine_printer.cpp.
• This change gates instantiation of engine methods to only the specializations that are compatible with the problem generator and avoids emitting invalid instantiations for unsupported engine/metric/dimension tuples.

Testing

• No automated tests were run as part of this change.

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Codex Task