feat(core): add Problem trait

The single trait users implement to describe an optimization problem:
associated `Decision: Clone` plus `objectives()` and `evaluate()`.
Both signatures match spec §8.1; `evaluate` takes `&self`.

src/core/mod.rs

@@ -4,6 +4,7 @@ pub mod candidate;
 pub mod evaluation;
 pub mod objective;
 pub mod population;
+pub mod problem;
 pub mod result;
 pub mod rng;
 
@@ -11,5 +12,6 @@ pub use candidate::*;
 pub use evaluation::*;
 pub use objective::*;
 pub use population::*;
+pub use problem::*;
 pub use result::*;
 pub use rng::*;