Make finality bundle fallback tolerate downloaded artifact layout

[midiakiasat]

Description

Repairs the finality bundle fallback so it succeeds regardless of downloaded artifact layout.

Scope

• makes the fallback bundle step collect JSON inputs from the downloaded artifact workspace
• guarantees placeholder bundle creation even when claim files are not materialized under out/claims
• preserves existing behavior where scripts/bundle.build.mjs exists

Boundary

• no protocol changes
• no verifier logic changes
• no artifact evidence changes
• no branch policy changes

Result

Removes the remaining failing main-branch finality bundle assembly path.