Release v0.3.1

- Maven Central publishing: pushing a v*.*.* tag now automatically signs and
  publishes the library to Maven Central via the CI pipeline (requires five
  CI/CD variables configured in GitLab — see .gitlab-ci.yml for details).
- Test suite hardened: all assertions use exact frame counts instead of
  greater-than-zero checks; flush loops fully drain the VP9 lookahead buffer
  before asserting output counts.