EvalFieldEvaluator

IFieldEvaluator wrapper around a compiled `::eval::Program`.

In the namespace construct::eval. Declared in src/libs/libconstruct/construct/eval/EvalFieldEvaluator.hpp.

Detailed description

Phase 6 of libeval-gpu-and-compute-migration (§EX2-MI-2). Replaces the parallel CPUFieldEvaluator (libspir) and NativeFieldEvaluator (libnative C++ JIT) paths with a single libeval-driven evaluator. The host builds an `::eval::Graph` from the SDFNode tree, compiles it once (Backend::Native preferred, Backend::Interpreted fallback), wraps the resulting Program in this class, and hands it to BoundsDetector / mesh-export / probe consumers.

Thread safety: each instance owns its own `::eval::ThreadCopy` and the underlying Program is shared via `std::shared_ptr` — multiple EvalFieldEvaluators built off the same Program may evaluate concurrently without synchronisation, mirroring the NativeFieldEvaluator's pure-fn-pointer thread model.