Meritocratic For You: rank by value, not audience size

[Pitchfork-and-Torch]

Summary

Makes For You favor predicted value for this viewer over how large an author's existing audience already is, and closes related preference/amplification holes.

This is individual meritocratic fairness (equity of attention), not demographic quotas. Quality still wins; negative scores are not lifted.

Changes

1. Author-size IPS — batch-mean-normalized Horvitz–Thompson residual on ln(1 + followers) so equal Phoenix scores are not ordered by follower count (docs/MERITOCRATIC_AUTHOR_SIZE_IPS.md).
2. Size-aware OON relief — softens the flat 0.75 out-of-network tax for small creators (full half-relief ≤1k followers, tapering to none by 100k) so discovery is not double-penalized by size leak + follow graph.
3. Origin-author diversity — author diversity decays on retweeted_user_id when present, so a viral original cannot flood the slate through many distinct retweeters.
4. Mute/block symmetry — quotes and reposts of muted authors are dropped the same way blocked ones already were.

Details and knobs: docs/FEED_FAIRNESS.md.

Test plan

• Unit coverage added for IPS, size-aware OON interpolation, origin-author diversity on/off, and mute of quoted/retweeted authors.
• home-mixer is a transparency dump without a Cargo workspace here; brace-balanced review of edited modules. Recommend CI / internal home-mixer test target when available.
• Feature flags default on but are independently disableable for A/B:
  • rust_home_mixer_enable_author_size_ips
  • rust_home_mixer_enable_size_aware_oon_relief
  • rust_home_mixer_enable_origin_author_diversity