feat(explorer): JSON export module + supporting metadata + example

Adds a tiny additive surface that turns any OptimizationResult into
a self-describing JSON file the heuropt-explorer webapp can load.
Real Pareto fronts have 50–200+ candidates spanning 2–7+ objectives;
reading them as numbers in a terminal scales badly. This commit
ships the heuropt-side of the explorer — the schema and the export
API. The webapp itself lives in a separate repo on its own cadence.

Three trait/type extensions, all with working defaults so existing
impls compile untouched:

- Objective gains optional `label: Option<String>` and
  `unit: Option<String>` fields, plus fluent builders
  `.with_label("Price").with_unit(\"\$k\")`. Existing
  `Objective::minimize(name)` / `Objective::maximize(name)` are
  unchanged. Both fields are #[serde(default,
  skip_serializing_if = \"Option::is_none\")] so existing JSON
  round-trips cleanly.
- Problem trait gains an optional
  `fn decision_schema(&self) -> Vec<DecisionVariable>` with default
  empty impl. Override it to provide pretty names / labels / units /
  bounds for the explorer; the default produces fallback x[0],
  x[1], … names. New DecisionVariable type at
  `heuropt::core::DecisionVariable` with builder methods.
- New `heuropt::traits::AlgorithmInfo` trait with `name()`
  (required) and `seed()` (default None). Every built-in algorithm
  — all 33 — implements it. Separate from Optimizer<P> so
  multi-fidelity Hyperband (which uses PartialProblem) implements
  it uniformly.

The new explorer module:

- `heuropt::explorer::ExplorerExport` envelope with versioned
  schema (SCHEMA_VERSION = 1).
- ExplorerCandidate per row, with front_rank from
  non_dominated_sort attached at export time so downstream tools
  don't re-derive it.
- ToDecisionValues adapter trait with provided impls for Vec<f64>,
  Vec<bool>, Vec<usize>, Vec<i64>; custom decision types implement
  one method.
- Free functions to_json / to_writer / to_file plus a builder API
  (with_algorithm_info, with_problem_name, with_wall_clock,
  with_timestamp).
- Gated on the existing `serde` feature, which now also pulls in
  `serde_json` as a dep.

The example:

- `examples/pick_a_car.rs` — promotes the README's PickACar to a
  real example, fully enriched with Objective labels/units and a
  decision_schema. Runs NSGA-III for 200 generations, prints a
  sample slice, writes pick_a_car.json. Gated on `serde`.

10 new explorer unit tests cover round-trip serde, fallback
decision-variable names, enriched export, AlgorithmInfo flow,
front-rank correctness, and the ToDecisionValues impls. Lib test
count went from 229 to 242.

src/algorithms/age_moea.rs

@@ -424,6 +424,15 @@ fn estimate_p(front_indices: &[usize], translated: &[Vec<f64>], m: usize) -> f64
     best_p
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for AgeMoea<I, V> {
+    fn name(&self) -> &'static str {
+        "AgeMoea"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/ant_colony_tsp.rs

@@ -429,6 +429,15 @@ fn better_than_so(
     }
 }
 
+impl crate::traits::AlgorithmInfo for AntColonyTsp {
+    fn name(&self) -> &'static str {
+        "AntColonyTsp"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/bayesian_opt.rs

@@ -540,6 +540,15 @@ impl BayesianOpt {
     }
 }
 
+impl crate::traits::AlgorithmInfo for BayesianOpt {
+    fn name(&self) -> &'static str {
+        "BayesianOpt"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/cma_es.rs

@@ -628,6 +628,15 @@ fn better_than_so(
     compare_so(a, b, direction) == std::cmp::Ordering::Less
 }
 
+impl crate::traits::AlgorithmInfo for CmaEs {
+    fn name(&self) -> &'static str {
+        "CmaEs"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/differential_evolution.rs

@@ -303,6 +303,15 @@ fn pick_three_distinct(
     (a, b, c)
 }
 
+impl crate::traits::AlgorithmInfo for DifferentialEvolution {
+    fn name(&self) -> &'static str {
+        "DifferentialEvolution"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/epsilon_moea.rs

@@ -396,6 +396,15 @@ fn box_dominates(a: &[i64], b: &[i64]) -> bool {
     strictly_less
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for EpsilonMoea<I, V> {
+    fn name(&self) -> &'static str {
+        "EpsilonMoea"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/genetic_algorithm.rs

@@ -317,6 +317,15 @@ fn compare_for_fitness<D>(
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for GeneticAlgorithm<I, V> {
+    fn name(&self) -> &'static str {
+        "GeneticAlgorithm"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/grea.rs

@@ -334,6 +334,15 @@ fn environmental_selection<D: Clone>(
     selected.into_iter().map(|i| combined[i].clone()).collect()
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Grea<I, V> {
+    fn name(&self) -> &'static str {
+        "Grea"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/hill_climber.rs

@@ -224,6 +224,15 @@ impl<I, V> HillClimber<I, V> {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for HillClimber<I, V> {
+    fn name(&self) -> &'static str {
+        "HillClimber"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/hype.rs

@@ -459,6 +459,15 @@ fn binary_tournament(fitness: &[f64], rng: &mut Rng) -> usize {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Hype<I, V> {
+    fn name(&self) -> &'static str {
+        "Hype"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/hyperband.rs

@@ -329,6 +329,19 @@ fn better(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     compare(a, b, direction) == std::cmp::Ordering::Less
 }
 
+impl<I, D> crate::traits::AlgorithmInfo for Hyperband<I, D>
+where
+    D: Clone,
+    I: Initializer<D>,
+{
+    fn name(&self) -> &'static str {
+        "Hyperband"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/ibea.rs

@@ -385,6 +385,15 @@ fn binary_tournament(fitness: &[f64], rng: &mut Rng) -> usize {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Ibea<I, V> {
+    fn name(&self) -> &'static str {
+        "Ibea"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/ipop_cma_es.rs

@@ -287,6 +287,15 @@ fn better(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     }
 }
 
+impl crate::traits::AlgorithmInfo for IpopCmaEs {
+    fn name(&self) -> &'static str {
+        "IpopCmaEs"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/knea.rs

@@ -328,6 +328,15 @@ fn perpendicular_distance(point: &[f64], extremes: &[usize], oriented: &[Vec<f64
     (dot - b).abs() / norm
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Knea<I, V> {
+    fn name(&self) -> &'static str {
+        "Knea"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/moead.rs

@@ -363,6 +363,15 @@ fn weight_distance(a: &[f64], b: &[f64]) -> f64 {
         .sqrt()
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Moead<I, V> {
+    fn name(&self) -> &'static str {
+        "Moead"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/mopso.rs

@@ -330,6 +330,15 @@ impl Mopso {
     }
 }
 
+impl crate::traits::AlgorithmInfo for Mopso {
+    fn name(&self) -> &'static str {
+        "Mopso"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/nelder_mead.rs

@@ -450,6 +450,12 @@ impl NelderMead {
     }
 }
 
+impl crate::traits::AlgorithmInfo for NelderMead {
+    fn name(&self) -> &'static str {
+        "NelderMead"
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/nsga2.rs

@@ -364,6 +364,15 @@ fn binary_tournament<D>(entries: &[Nsga2Entry<D>], rng: &mut Rng) -> usize {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Nsga2<I, V> {
+    fn name(&self) -> &'static str {
+        "Nsga2"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/nsga3.rs

@@ -542,6 +542,15 @@ fn associate(
     (assoc, dist)
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Nsga3<I, V> {
+    fn name(&self) -> &'static str {
+        "Nsga3"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/one_plus_one_es.rs

@@ -285,6 +285,15 @@ impl OnePlusOneEs {
     }
 }
 
+impl crate::traits::AlgorithmInfo for OnePlusOneEs {
+    fn name(&self) -> &'static str {
+        "OnePlusOneEs"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/paes.rs

@@ -242,6 +242,15 @@ impl<I, V> Paes<I, V> {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Paes<I, V> {
+    fn name(&self) -> &'static str {
+        "Paes"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/particle_swarm.rs

@@ -357,6 +357,15 @@ fn best_index(values: &[f64], direction: Direction) -> usize {
     idx
 }
 
+impl crate::traits::AlgorithmInfo for ParticleSwarm {
+    fn name(&self) -> &'static str {
+        "ParticleSwarm"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/pesa2.rs

@@ -409,6 +409,15 @@ fn truncate_by_grid<D: Clone>(archive: &mut ParetoArchive<D>, max_size: usize, d
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for PesaII<I, V> {
+    fn name(&self) -> &'static str {
+        "PesaII"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/random_search.rs

@@ -158,6 +158,15 @@ impl<I> RandomSearch<I> {
     }
 }
 
+impl<I> crate::traits::AlgorithmInfo for RandomSearch<I> {
+    fn name(&self) -> &'static str {
+        "RandomSearch"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/rvea.rs

@@ -466,6 +466,15 @@ fn smallest_neighbor_angle(references: &[Vec<f64>]) -> f64 {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Rvea<I, V> {
+    fn name(&self) -> &'static str {
+        "Rvea"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/simulated_annealing.rs

@@ -333,6 +333,15 @@ impl<I, V> SimulatedAnnealing<I, V> {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for SimulatedAnnealing<I, V> {
+    fn name(&self) -> &'static str {
+        "SimulatedAnnealing"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/sms_emoa.rs

@@ -289,6 +289,15 @@ fn pick_drop_index<D>(
     worst_front[worst_idx_in_front]
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for SmsEmoa<I, V> {
+    fn name(&self) -> &'static str {
+        "SmsEmoa"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/snes.rs

@@ -389,6 +389,15 @@ fn better(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     compare(a, b, direction) == std::cmp::Ordering::Less
 }
 
+impl crate::traits::AlgorithmInfo for SeparableNes {
+    fn name(&self) -> &'static str {
+        "SeparableNes"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/spea2.rs

@@ -503,6 +503,15 @@ fn binary_tournament(fitness: &[f64], rng: &mut Rng) -> usize {
     }
 }
 
+impl<I, V> crate::traits::AlgorithmInfo for Spea2<I, V> {
+    fn name(&self) -> &'static str {
+        "Spea2"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/tabu_search.rs

@@ -331,6 +331,20 @@ where
     }
 }
 
+impl<D, I, N> crate::traits::AlgorithmInfo for TabuSearch<D, I, N>
+where
+    D: Clone + Hash + Eq,
+    I: Initializer<D>,
+    N: FnMut(&D, &mut Rng) -> Vec<D>,
+{
+    fn name(&self) -> &'static str {
+        "TabuSearch"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/tlbo.rs

@@ -325,6 +325,15 @@ fn better(a: &Evaluation, b: &Evaluation, direction: Direction) -> bool {
     }
 }
 
+impl crate::traits::AlgorithmInfo for Tlbo {
+    fn name(&self) -> &'static str {
+        "Tlbo"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/tpe.rs

@@ -479,6 +479,15 @@ impl Tpe {
     }
 }
 
+impl crate::traits::AlgorithmInfo for Tpe {
+    fn name(&self) -> &'static str {
+        "Tpe"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/algorithms/umda.rs

@@ -352,6 +352,15 @@ fn better_than_so(
     compare_so(a, b, direction) == std::cmp::Ordering::Less
 }
 
+impl crate::traits::AlgorithmInfo for Umda {
+    fn name(&self) -> &'static str {
+        "Umda"
+    }
+    fn seed(&self) -> Option<u64> {
+        Some(self.config.seed)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;