Add upstream Radar social candidates

[yvette-carlisle]

Summary

• refresh the upstream openai/codex Radar review queue
• add source-backed reviews, impacts, bundles, and social_candidate/v1 artifacts for PRs 25469, 26106, and 26254
• add social_candidate/v1 spec and Radar validation support so source-analysis automation can hand off Publisher opportunities without writing social_post/v1

Validation

• cargo make fmt-rust
• cargo test -p decodex accepts_valid_social_candidate_and_rejects_missing_refs
• cargo test -p decodex accepts_valid_upstream_review_and_rejects_bad_action
• cargo run -p decodex --bin decodex -- radar validate artifacts/github/review-queue artifacts/github/reviews artifacts/github/impact artifacts/github/social-candidates artifacts/github/bundles
• cargo make check-site-content
• cargo make fmt-check
• cargo make check-rust

Boundaries

• no X publishing
• no social_post/v1 writes
• no Linear mutation