feat: add multi-objective optimization with NSGA-II

Add MultiObjectiveStudy for optimizing multiple objectives simultaneously,
backed by NSGA-II (Non-dominated Sorting Genetic Algorithm II) with SBX
crossover, polynomial mutation, and constraint-aware dominance.

New public API:
- MultiObjectiveStudy with optimize(), pareto_front(), ask()/tell()
- MultiObjectiveTrial with get(), is_feasible(), user attributes
- MultiObjectiveSampler trait for custom MO samplers
- Nsga2Sampler with builder for population size, crossover/mutation params
- ObjectiveDimensionMismatch error variant

src/error.rs

@@ -76,6 +76,15 @@ pub enum Error {
     #[error("trial was pruned")]
     TrialPruned,
 
+    /// Returned when the objective returns the wrong number of values.
+    #[error("objective dimension mismatch: expected {expected} values, got {got}")]
+    ObjectiveDimensionMismatch {
+        /// The expected number of objective values.
+        expected: usize,
+        /// The actual number of objective values returned.
+        got: usize,
+    },
+
     /// Returned when an internal invariant is violated.
     #[error("internal error: {0}")]
     Internal(&'static str),