feat(core): add data types and Rng alias

Plain-data structs and the seeded Rng alias from spec §7. Each lives in
its own file under src/core/ with unit tests:

- Direction, Objective, ObjectiveSpace (with as_minimization negating
  only Maximize axes)
- Evaluation (is_feasible == constraint_violation <= 0.0)
- Candidate<D>, Population<D> (concrete, public fields, From<Vec<...>>)
- OptimizationResult<D>
- type Rng = rand::rngs::StdRng + rng_from_seed, so no public trait is
  generic over the RNG (spec §2.5)

All public types behind #[cfg_attr(feature = "serde", derive(...))] so
the optional feature wires up without changing the default surface.

src/core/mod.rs

@@ -0,0 +1,15 @@
+//! Concrete data types and the `Problem` trait that the rest of the crate is built on.
+
+pub mod candidate;
+pub mod evaluation;
+pub mod objective;
+pub mod population;
+pub mod result;
+pub mod rng;
+
+pub use candidate::*;
+pub use evaluation::*;
+pub use objective::*;
+pub use population::*;
+pub use result::*;
+pub use rng::*;