v0.9.1

No functional changes.

• Every GitHub Actions workflow runs read-only by default, and each action is
  pinned to a commit rather than a tag that can be moved.
• The development container pins its base image by digest and every package by
  version, so a Linux build compiles against the same toolchain each time.
• Static analysis through CodeQL, a published coverage figure, and an OpenSSF
  Scorecard run, alongside the sanitizers, clang-tidy, and fuzzing already here.
• Release binaries carry their build provenance attestation on the release
  itself, so a signature can be checked beside the file it covers.
• A security policy, at SECURITY.md.