Author diversity resets on every For You request, so the same author repeats across consecutive loads

[aunysillyme]

Problem
The same author can fill For You across consecutive refreshes and scrolls, because author diversity is computed per request and starts from zero each time.

Current behavior (6bb4594)

• author_pool_counts counts an author's posts only among the current request's candidates:
  https://github.com/xai-org/x-algorithm/blob/6bb4594/home-mixer/scorers/ranking_scorer.rs#L629
• AuthorDiversityDecay = 0.5, AuthorDiversityFloor = 0.25 (2nd post from an author in a request scored at 0.625, 3rd at 0.4375):
  https://github.com/xai-org/x-algorithm/blob/6bb4594/home-mixer/params/param.rs#L229-L245
• The query carries served_ids (post IDs only), and PreviouslyServedPostsFilter uses them to drop exact posts already served, not authors:
  https://github.com/xai-org/x-algorithm/blob/6bb4594/home-mixer/models/query.rs#L49
  https://github.com/xai-org/x-algorithm/blob/6bb4594/home-mixer/filters/previously_served_posts_filter.rs#L26-L30

Result: an author's first post in every new load gets a full 1.0 multiplier, even if the viewer was shown that author several times in the previous load seconds earlier.

Proposal

1. Carry recently served author IDs (bounded, e.g. last N requests or last few minutes) with the query.
2. Seed author_pool_counts with those counts, time-decayed, so diversity continues across loads instead of resetting.
3. Ship behind a param, off by default, for controlled evaluation.

Related

• Add experimental slate-level diversity reranking #37: hard author cap within a single slate (off by default)
• docs: propose cross-request semantic fatigue experiment #83: cross-request semantic fatigue (topic-keyed, not author-keyed)
• Stop RankingScorer author diversity from keying the retweeter #172 (closed): diversity keyed on the retweeter

Open questions

• Window size and decay rate
• Whether mutual follows or authors the viewer engages with heavily should get a softer penalty