Stop Phoenix authorIsNsfwSeq from skipping retweets

[Pitchfork-and-Torch]

Phoenix hashes get_original_author_id() but as_tweet_info zeroed safety_label_mask on every retweet. Gizmoduck already fetched the origin user and then wrote the retweeter's NSFW bit into nsfw_author_phoenix. Training reads safetyLabelMaskSeq bit 2; serving therefore scored every RT of an NSFW original as a clean-author post.

Proof

• Entry: GizmoduckCandidateHydrator (origin user already fetched) then PostCandidate::as_tweet_info
• Sink: PhoenixScorer -> InputBuffer authorIsNsfwSeq from TweetInfo.safety_label_mask bit 2
• Break: retweeted_user_id.is_none() guard forced mask 0; phoenix bit keyed the retweeter
• Viewer: in-network RTs of NSFW originals rank as if the origin author were clean
• Twin: tweet_id and author_id already use the origin; originals already stamp nsfw_author_phoenix; history UAS stamps the engaged tweet mask

This is not #125 (size residual, quoted OR, gizmoduck fail-closed). Not #185 (OON NSFW pre-score filter). Not #172/#188 (diversity/MoE retweeter key). Not #173 (safemodel tweet labels). Not #189 (DedupConversationFilter). Filter and ads poster bits are unchanged.

Fix

• nsfw_author_phoenix on a retweet is the origin author's bit
• as_tweet_info stamps that bit on retweets too

Tests cover origin-NSFW RT stamp, retweeter-NSFW clean-origin no stamp, and original-post unchanged.

cargo test cannot run home-mixer. Public dump has no Home Mixer manifest. Standalone rustc model of old vs new stamp: 5/5 passed.

Fork PR: none

[Pitchfork-and-Torch]

Parking this intern-stack PR per operator GitHub cleanup (2026-09-08). Hunt notes remain local. Not a reject of the class.