Help wanted: add figure caption diagnostics

[zxyasfas]

User pain

Users need to know which figure caption failed and what to fix. A score drop without location or evidence is hard to act on.

Expected PR

• Add or improve one figure-caption diagnostic.
• Include evidence or location fields when available.
• Add a synthetic failing case that proves the diagnostic is emitted.

Acceptance criteria

• The report explains what failed.
• The report gives an actionable fix.
• Tests cover the changed behavior.
• Standard validation commands pass.

Privacy

Use synthetic figure captions only. Do not include real papers, images, or private format guides.