Add CI across supported Bazel and host platform versions

[darthfork]

Official-feeling rulesets have a clear support matrix and enforce it continuously.

Tasks:

• Define supported Bazel versions.
• Add CI for at least latest Bazel LTS plus current release.
• Add CI on macOS and Linux.
• Run examples and tests in CI.

Acceptance criteria:

• CI status is meaningful and required for merges.
• Support matrix is documented.

Parent issue: #4