Stop IneligibleSubscriptionFilter from dropping exclusive posts on a subscription-list miss

[Pitchfork-and-Torch]

Bug

For You IneligibleSubscriptionFilter drops exclusive posts when the subscription list was never loaded, and when the viewer is the conversation author.

SubscribedUserIdsQueryHydrator errors are ignored by hydrate_query. The list stays empty. The mixer filter treated empty as "subscribed to nobody" and dropped every subscription_author_id: Some(...) before scoring.

VF DropExclusiveTweetContentRule allows the conversation author. The mixer filter did not.

• Entry: Phoenix IneligibleSubscriptionFilter (phoenix_candidate_pipeline.rs)
• Sink: pre-scoring drop of exclusive / Super Follow posts
• Break: socialgraph miss → empty list → drop all exclusive; conversation author never exempted
• Viewer effect: paying subscribers lose exclusive posts on a list-RPC miss; conversation authors lose exclusive replies in For You
• Twin: VF still decides exclusive access when the mixer list is missing

This is not #96–#120 (VF, mute/block, quote, scarecrow, geo). Brazil 2026 filter is not in this snapshot.

Fix

Mark the subscription list hydrated only after a successful socialgraph read. Keep exclusive posts when the list was not loaded. Keep exclusive posts when the viewer is the conversation author.

A hydrated empty list still drops exclusive posts for everyone except the conversation author.

Tests

Filter unit tests cover hydrator-miss keep, conversation-author keep, subscribed keep, unsubscribed drop, and hydrated-empty drop. Hydrator tests cover success marking the list ready and errors leaving it unready.

cargo: cannot run the Home Mixer crate here. Public dump has no Home Mixer manifest. Standalone keep-rule check: 6/6 passed.

[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.