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.

docs/book/src/cookbook/custom-optimizer.md

@@ -134,8 +134,11 @@ parallel.
   result.
 - **No error type.** Invalid configuration panics with a clear
   message; this matches the style of the built-in algorithms.
-- **No async.** `evaluate` is synchronous; for async work, drive it
-  on a tokio runtime around the optimizer loop yourself.
+- **No async on the trait.** `Optimizer<P>` is synchronous. For
+  async evaluation, implement [`AsyncProblem`](https://docs.rs/heuropt/latest/heuropt/core/async_problem/trait.AsyncProblem.html)
+  on your problem and use the `run_async(&problem, concurrency)`
+  method that comes with the `async` feature. See the
+  [Async evaluation cookbook recipe](./async.md).
 
 The smallness is the point: you should be able to read a built-in
 algorithm and write your own in an afternoon. See

docs/book/src/cookbook/parallel.md

@@ -9,7 +9,7 @@ population, and rayon parallelizes that batch.
 
 ```toml
 [dependencies]
-heuropt = { version = "0.5", features = ["parallel"] }
+heuropt = { version = "0.8", features = ["parallel"] }
 ```
 
 There's nothing else to opt into in your code. The
@@ -104,6 +104,16 @@ to scope it.
   parallelism rarely helps.
 - The algorithm is steady-state (Paes, SA, hill climber).
 
+## `parallel` vs `async`
+
+| If your `evaluate` is… | Use |
+|---|---|
+| CPU-bound (math, simulation) | `parallel` feature (this recipe) |
+| IO-bound (HTTP, RPC, subprocess) | `async` feature → see [Async evaluation](./async.md) |
+
+Both can be on at once if your evaluation does *both* substantial
+CPU work *and* IO. The two features are independent.
+
 [`RandomSearch`]: https://docs.rs/heuropt/latest/heuropt/algorithms/random_search/struct.RandomSearch.html
 [`Nsga2`]: https://docs.rs/heuropt/latest/heuropt/algorithms/nsga2/struct.Nsga2.html
 [`Nsga3`]: https://docs.rs/heuropt/latest/heuropt/algorithms/nsga3/struct.Nsga3.html