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/choosing-an-algorithm.md

@@ -222,9 +222,15 @@ evaluate via rayon when the feature is on. **Seeded runs stay
 bit-identical** to serial mode.
 
 ```toml
-heuropt = { version = "0.5", features = ["parallel"] }
+heuropt = { version = "0.8", features = ["parallel"] }
 ```
 
+If your evaluation is **IO-bound** (HTTP request, RPC, subprocess)
+rather than CPU-bound, use the `async` feature instead — it gives
+you `AsyncProblem` and a `run_async(&problem, concurrency).await`
+method on every algorithm in the catalog. See the
+[Async evaluation cookbook recipe](./cookbook/async.md).
+
 ## TL;DR table
 
 | Situation | Pick |