Feature flags and execution strategies

The crate exposes the following feature flags:

• cpu includes the CPU backend in the default feature set.
• metrics exposes metrics emission from hot paths.
• gpu prepares the GPU execution path selection surface (the accelerator implementation is not yet available).
• skeleton is a legacy compatibility flag retained for early versions; it is no longer required by the CPU backend.

Choose an ExecutionStrategy that matches the compiled features. Allowing Auto keeps behaviour stable across builds while seamlessly adopting GPU support when available.