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/candidate.rs

@@ -0,0 +1,35 @@
+//! A decision paired with its evaluation.
+
+use crate::core::evaluation::Evaluation;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// A decision together with its evaluated objective values.
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Debug, Clone, PartialEq)]
+pub struct Candidate<D> {
+    /// The decision (input to the problem).
+    pub decision: D,
+    /// The evaluated objective values and constraint violation.
+    pub evaluation: Evaluation,
+}
+
+impl<D> Candidate<D> {
+    /// Pair a decision with its evaluation.
+    pub fn new(decision: D, evaluation: Evaluation) -> Self {
+        Self { decision, evaluation }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn new_pairs_decision_and_evaluation() {
+        let c = Candidate::new(vec![1.0, 2.0], Evaluation::new(vec![5.0]));
+        assert_eq!(c.decision, vec![1.0, 2.0]);
+        assert_eq!(c.evaluation.objectives, vec![5.0]);
+    }
+}