Documentation-only patch release over 9.0.1.
- `@version` in mix.exs moves 9.0.1 -> 9.0.2.
- CHANGELOG.md gains a dated `## [9.0.2]` section describing the
README rework, the new statement-surface example, the move onto the
family's canonical example domains, and the three stale facts fixed
in docs/architecture.md.
The README pin stays `{:predicator, "~> 9.0"}`: it carries no patch
component, so a patch release leaves it alone.
No changelog fragments were assembled - changelog.d/README.md excludes
documentation, so the two commits in this release wrote none, and the
section above is authored at release time the way 9.0.1's was.
Refs: px-k1l