Recompute mutual-follow bits on cached For You posts

[Pitchfork-and-Torch]

Bug

BidirectionalFollowHydrator.enable was flag && !has_cached_posts.

FollowedUserIdsQueryHydrator still runs on a cache hit. The follow list is current. RankingScorer still runs. It reads is_mutual_follow_author and, when that bit is Some(true), adds BidirectionalFollowReplyWeightBoost to the reply head. That boost defaults to 15.0 on a ReplyWeight of 5.0.

Skipping the hydrator keeps the bit written up to 180s earlier. A new mutual does not get the boost. A broken mutual keeps a 4x reply-head weight. PhoenixScorer is also skipped on the cache hit, so the weighted score is rebuilt from cached phoenix heads times the stale bit.

CachedPostsQueryHydrator sets has_cached_posts when Redis returns at least 500 posts. TTL is 180s.

This is not #135 (in_network bit). Not #156 (block / nsfw_author / quote).

Five-line proof

• Entry: BidirectionalFollowHydrator (enable skipped has_cached_posts)
• Sink: RankingScorer::reply_weight_for / dwell_weight_for (is_mutual_follow_author == Some(true))
• Break: cache hit left the Redis slate bit in place while the follow list was live
• Viewer effect: originals from a new mutual stay unboosted; originals from a broken mutual keep a 4x reply-head boost for up to 180s
• Twin: Recompute in_network on cached For You posts #135 recomputes in_network on the same cache hit; Rehydrate block and author-label bits on cached For You posts #156 recomputes block / nsfw bits

Fix

Enable whenever the hydration flag is on. update overwrites the cached bits.

Tests

• enable is true on cache hit and miss
• cache-hit hydrate + update: author no longer follows viewer -> Some(false)
• cache-hit hydrate + update: viewer unfollowed -> Some(false)
• cache-hit hydrate + update: newly mutual -> Some(true)

RankingScorer test bidirectional_weight_boosts_only_mutual_original_posts is the score sink.

cargo test cannot run here. Public dump has no Home Mixer manifest.