Publish clean-machine verification preview

[henry8minus1]

What changed

Publishes a fresh scrubbed export from private source revision 0e4886a7f514386f7a6b5f7cf261332778e9db8b for the Level B clean-VM trial.

Includes:

• twelve metadata-sanitized M42 sample frames with CC BY 4.0 attribution;
• manifest and pixel/file checksums;
• public_free_core workflow and CLI selection support;
• public sample and release-smoke validators;
• public GitHub Actions smoke workflow;
• release provenance and SHA256SUMS;
• current sanitized NOVA source and workflow history.

Verification

• Export scrub scan: PASS.
• Structural public-release smoke: PASS.
• Sample checksums/header allowlist: PASS.
• 74 Python source files byte-compile: PASS.
• Copied tree matches the generated export byte-for-byte.

Known incomplete evidence

This remains an explicitly preliminary preview. sample_data/golden.json is pending_baseline; its zero values are placeholders, not claimed tolerances. End-to-end reproducibility remains pending Jeff Henry’s clean-VM trial and second-machine evidence. No verified release tag should be created from this PR alone.