GitHub Actions build matrix + checksums #16
Description
Summary
- GitHub Actions build matrix and checksums.
Context
- CI must validate tests and produce reproducible artifacts.
Scope
- Matrix builds for all targets with smoke tests.
- Produce SHA256SUMS for release artifacts.
- Use pinned action SHAs and mise tasks.
Out of scope
- Release automation and signing (later).
Acceptance criteria
- CI builds pass across all targets and emit checksums.