Allow missing artifacts for terminal format failures

[Anionix]

Merged review finding:

• Package complete release evidence #83 (comment)

prepare_run records an artifact path before encoding. FAILED and UNSUPPORTED formats therefore retain a path even though no file exists, while release packaging currently requires every manifest artifact unconditionally.

Acceptance:

• missing manifest artifacts are allowed only for FAILED or UNSUPPORTED format entries
• ENCODED, ROUNDTRIP_VERIFIED, BENCHMARKED, and REPORTED entries still require their artifact
• present terminal-failure artifacts remain packageable
• claims and prompt artifact requirements remain strict
• regression tests cover both allowed terminal absence and rejected active absence