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/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::*;