Stop For You VF from skipping Phoenix reply parents

[Pitchfork-and-Torch]

Bug

Phoenix retrieval, MOE, Topics, and TweetMixer set in_reply_to_tweet_id and leave ancestors empty. Thunder and Latest Following copy the parent into ancestors.

VFCandidateHydrator only fetches candidate.ancestors as ancillary ids. should_drop_ancillary only walks that list. A mute or block on the parent tweet never reaches AncillaryVFFilter, so the reply still ranks on For You.

This is not conversation-gap expansion (#97). Not mixer ancestor_users mute/block (#101 / #111). Not quote/RT handle mute-keyword (#140 / #164). Not civic followee quotes (#178). Not seen/served reply parents (#179).

Fix

When ancestors is empty, treat in_reply_to_tweet_id as the reply parent for VF fetch and ancillary drop. Thunder's existing ancestor list is unchanged.

Proof

• Entry: PhoenixSource / PhoenixMOESource / PhoenixTopicsSource / TweetMixerSource write in_reply_to_tweet_id and leave ancestors empty
• Sink: VFCandidateHydrator oon_ids + should_drop_ancillary + AncillaryVFFilter
• Break: ancillary VF walked ancestors only; OON Phoenix replies never populated that list
• Viewer effect: reply to a muted or blocked parent still serves on For You
• Twin: ThunderSource and Following Night Owl already push in_reply_to into ancestors

Tests

• reply_ancestor_ids_uses_in_reply_to_when_ancestors_empty
• reply_ancestor_ids_keeps_thunder_ancestors
• phoenix_reply_drops_when_parent_author_is_muted
• phoenix_reply_drops_when_parent_author_blocked_viewer
• phoenix_reply_keeps_when_parent_is_allowed
• thunder_ancestors_still_drop_and_do_not_use_a_stale_in_reply_to
• unit tests added; cargo test cannot run in the public dump (no home-mixer Cargo.toml)

[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.