Bump to 0.2.0 and add crates.io keywords/categories

Resolves the version/changelog mismatch flagged while checking publish
readiness: Cargo.toml had sat at 0.1.0 through everything since the initial
commit, while CHANGELOG.md's `[0.1.0] - 2026-10-23` entry called itself
"Initial release" with no git tag behind it and nothing ever actually
published (crates.io 404s on the name). Rather than relabel that entry after
the fact, it stays as the historical record it is; `[Unreleased]` becomes
`[0.2.0] - 2026-10-25` instead, with a fresh empty `[Unreleased]` left above
it for whatever comes next.

`categories` and `keywords` were empty, which is legal but leaves the crate
unreachable from crates.io's own browse and search pages. Checked both
category slugs against `/api/v1/category_slugs` rather than guessing —
`web-programming::http-client` in particular is a subcategory that doesn't
show up in the plain `/categories` listing, only in the slug list.

Re-ran `cargo publish --dry-run` after both changes: packages, verifies in
an isolated build, and reaches the upload step with no manifest warnings.
Also re-checked the full suite, clippy, fmt, the MSRV shell, and
`nix flake check` — all pass under the bumped version.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.2.0] - 2026-10-25
+
 ### Added
 
 - E-invoicing, seventeen methods: `validate_einvoice`,