Stop seen/served filters from dropping replies to seen parents

[Pitchfork-and-Torch]

Bug

related_post_ids_iter chained in_reply_to_tweet_id next to the candidate id and the retweet original. PreviouslySeen, PreviouslySeenBackup, and PreviouslyServed hard-drop a candidate when any related id is in seen / impressed / served.

A reply is new speech. Seeing or serving the parent is not seeing the reply. Followee replies to a post the viewer already scrolled past never reach For You.

This is not #151 (quote same-card). #151 adds quoted_tweet_id. This removes the parent id. A retweet of a seen original still drops.

Five-line proof

• Entry: TES / Thunder in_reply_to_tweet_id on an in-network reply
• Sink: PreviouslySeen / PreviouslySeenBackup / PreviouslyServed (related_post_ids_iter)
• Break: parent id was treated as the reply card
• Viewer effect: a followee reply to a seen/served/impressed post disappears from For You
• Twin: same reply kept when the parent is absent from those sets; RT of a seen original still drops

Fix

Same-card ids are the post and its retweet original only.

Tests

• related_ids_are_the_card_and_retweet_original_only
• keeps_reply_when_only_the_parent_was_seen
• drops_reply_when_the_reply_itself_was_seen
• keeps_reply_when_only_parent_was_impressed
• keeps_reply_when_only_the_parent_was_served

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

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.