docs(0.9): release notes, cookbook recipe, README polish

Companion to the feat(explorer) commit. Bumps the version and
brings every cross-referencing doc up to v0.9 currency.

- Cargo.toml: version 0.8.0 -> 0.9.0.
- CHANGELOG: 0.9.0 entry covering the explorer export, the
  Problem-side metadata additions, the AlgorithmInfo trait, the
  pick_a_car example, and the new cookbook recipe.
- README: closing paragraph of the PickACar example points users
  at the explorer with a one-call snippet
  (`ExplorerExport::from_result(...).with_algorithm_info(...)
  .to_file(...)?`). Version snippets bumped 0.8 -> 0.9.
- New cookbook recipe at docs/book/src/cookbook/explorer.md
  covering: enabling the serde feature, enriching Problem with
  labels/units/decision-schema, the export call, the JSON schema,
  and custom decision-type handling.
- SUMMARY.md and cookbook.md link the new recipe.
- migration.md: new "To 0.9" section documenting the additive
  changes (purely backwards-compatible upgrade from 0.8.x).
- introduction.md, comparison.md, choosing-an-algorithm.md,
  stability.md: version refs bumped 0.8 -> 0.9.
- cookbook/parallel.md, cookbook/async.md: version refs bumped
  0.8 -> 0.9.
- getting-started.md: version refs bumped, serde feature
  description expanded to mention the explorer module.
- SECURITY.md: supported-versions table moves to 0.9.x.

docs/book/src/cookbook.md

@@ -14,17 +14,21 @@ project.
   optimizer await many evaluations concurrently. The differentiating
   feature vs other optimization libraries.
 - [Tune a model with expensive evaluations](./cookbook/expensive-evaluations.md)
-  — `BayesianOpt`, `Tpe`, and `Hyperband` for the 50–500-eval
+  — Bayesian Optimization, TPE, and Hyperband for the 50–500-eval
   regime.
 - [Compare two algorithms on your problem](./cookbook/compare.md) —
   multi-seed harness pattern straight from `examples/compare.rs`.
 - [Optimize a permutation (TSP-style)](./cookbook/permutation.md) —
-  `AntColonyTsp` with a distance matrix.
+  Ant Colony with a distance matrix.
 - [Constrain your search with `Repair`](./cookbook/constraints.md) —
   bounds, simplex projection, custom repair.
 - [Pick one answer off a Pareto front](./cookbook/pick-one.md) — the
   a-posteriori weighted-decision pattern from the `jiggly_tuning`
   example.
+- [Explore your results in a webapp](./cookbook/explorer.md) — export
+  an `OptimizationResult` to JSON and browse it interactively at
+  [heuropt-explorer](https://swaits.github.io/heuropt-explorer/) —
+  parallel coordinates, scatter, range filters, weighted ranking.
 - [Write your own algorithm](./cookbook/custom-optimizer.md) —
   implement `Optimizer<P>` from scratch, à la the
   `examples/custom_optimizer.rs` walkthrough.