docs: 0.8.0 release polish — README, guide, changelog

Companion to the feat(async) commit. Brings every cross-referencing
doc up to v0.8 currency, replaces marketing-flavored copy with plain
prose, and replaces toy benchmark problems with relatable ones that
include actual run output and interpretive narrative.

- README: collapses the four-bullet "Read the user guide / API
  reference / Tested with N tests / Hot paths optimized" list into
  a single Docs links line.
- README: replaces the Schaffer-N1 toy problem with a PickACar
  multi-objective design problem — three decision variables
  (displacement, weight, drag), four objectives (price, 0-60,
  fuel, noise), and *nonlinear* cost relationships so the Pareto
  front is a real surface, not a 1D sweep. Includes actual NSGA-III
  run output (representative slice across the 100-car front) and
  a narrative explaining what each row tells you and why hand-
  picking would miss the interesting tradeoffs.
- README: removes rustdoc-style hidden `#` setup lines from code
  blocks. The README is rendered as plain markdown on GitHub /
  crates.io, where those lines are visible garbage instead of
  hidden setup. Code blocks are now self-contained.
- Guide quickstart (getting-started.md): replaces Sphere ( Σ x² )
  with a least-squares LineFit example. Same shape (single-
  objective continuous), but recognizable framing. Includes
  actual CMA-ES output, residual table, and narrative comparing
  the answer to standard regression.
- Algorithm count audit: stale "35 algorithms" claim corrected to
  the actual 33 across README, src/lib.rs, introduction.md, and
  the comparison.md table cell.
- Async feature flag listed in the optional-features sections of
  README, src/lib.rs, getting-started.md.
- introduction.md, choosing-an-algorithm.md, comparison.md,
  stability.md, migration.md, cookbook/parallel.md,
  cookbook/custom-optimizer.md: cross-references updated to
  describe full async coverage and link the new cookbook recipe.
- stability.md: removes the speculative "Observer / Snapshot /
  Checkpoint planned" bullet (those didn't ship); documents the
  AsyncProblem / AsyncPartialProblem trait stability.
- migration.md: new "To 0.8" section with paths from 0.5.x and 0.7.x.
- CHANGELOG: 0.8.0 entry capturing the async feature plus the
  documentation / governance / CI catch-up.
- SECURITY.md: supported versions table reflects 0.8.x.

src/lib.rs

@@ -4,7 +4,7 @@
 //! The crate aims to make three things obvious:
 //!
 //! 1. **Define a problem** by implementing [`Problem`](crate::core::Problem).
-//! 2. **Run a built-in optimizer** — pick from 35 algorithms in
+//! 2. **Run a built-in optimizer** — pick from 33 algorithms in
 //!    [`algorithms`] covering single-objective continuous (CMA-ES,
 //!    Differential Evolution, Nelder-Mead, …), multi-objective
 //!    (NSGA-II, MOPSO, IBEA, MOEA/D, …), many-objective (NSGA-III,
@@ -32,6 +32,12 @@
 //! - `parallel` — rayon-backed parallel population evaluation in
 //!   every population-based algorithm. Seeded runs stay bit-
 //!   identical to serial mode.
+//! - `async` — adds the
+//!   [`AsyncProblem`](crate::core::async_problem::AsyncProblem) and
+//!   [`AsyncPartialProblem`](crate::core::async_problem::AsyncPartialProblem)
+//!   traits and a `run_async(&problem, concurrency).await` method on
+//!   every algorithm. Use this when your `evaluate` does IO (HTTP,
+//!   RPC, subprocess) — see the [Async evaluation cookbook recipe](https://swaits.github.io/heuropt/cookbook/async.html).
 //!
 //! # Quick example
 //!