2.1.9

chore(release): bump version to 2.1.9

Update version strings across the repository:
- .version, Akku.manifest, run.ss fallback
- Dockerfile and Dockerfile.musl default VERSION build-arg
- build.sh fallback and GitHub Actions workflow fallbacks
- README.md and doc/release-history.md release notes

Release 2.1.9 highlights the match-index-node/ufo-match-steer
migration: the analyzer shrank by ~1,100 lines (~5%) and analysis is
32-51% faster (init-workspace 42.4s -> 23.2s without type inference,
64.5s -> 34.1s with type inference).