feat(algorithms): add NelderMead simplex direct-search optimizer

Nelder & Mead 1965: gradient-free local optimizer that maintains a
simplex of n+1 points in n-D and at each iteration replaces the worst
vertex by one of {reflect, expand, outside-contract, inside-contract,
shrink} relative to the centroid of the rest. The five standard
coefficients (reflection α=1, expansion γ=2, contraction ρ=0.5,
shrinkage σ=0.5) are exposed in the config but default to canonical
values so users can leave them alone.

Single-objective only, Vec<f64> only, bounds enforced by clamping
each new vertex. Termination is purely iteration-count for v0.2;
"vertices have collapsed" stopping is a future enhancement.

Filling a real gap: heuropt had population-based local search
(SimulatedAnnealing, HillClimber) but no classical direct-search
algorithm. Excellent for low-dim smooth-ish problems where a
population is overkill.

src/prelude.rs

@@ -25,7 +25,8 @@ pub use crate::algorithms::{
     AgeMoea, AgeMoeaConfig, AntColonyTsp, AntColonyTspConfig, CmaEs, CmaEsConfig, DifferentialEvolution,
     DifferentialEvolutionConfig, EpsilonMoea, EpsilonMoeaConfig,
     GeneticAlgorithm, GeneticAlgorithmConfig, Grea, GreaConfig, HillClimber, HillClimberConfig, Hype,
-    HypeConfig, Ibea, IbeaConfig, Knea, KneaConfig, Moead, MoeadConfig, Mopso, MopsoConfig, Nsga2,
+    HypeConfig, Ibea, IbeaConfig, Knea, KneaConfig, Moead, MoeadConfig, Mopso, MopsoConfig,
+    NelderMead, NelderMeadConfig, Nsga2,
     Nsga2Config, Nsga3, Nsga3Config, OnePlusOneEs, OnePlusOneEsConfig, Paes, PaesConfig, ParticleSwarm, PesaII, PesaIIConfig,
     ParticleSwarmConfig, RandomSearch, RandomSearchConfig, Rvea, RveaConfig,
     SimulatedAnnealing,