Stop Explore from ranking posts when topic hydration is missing

[Pitchfork-and-Torch]

Problem

Bulk Explore (topic_ids.len() > 6) is an exclusion filter. A missing topic hydration (filtered_topic_ids = None) was treated as keep. FilteredTopicsHydrator also wrote None on a successful empty lookup, so a Strato miss and a real untagged post were the same value.

TopicIdsFilter runs before VF. Withheld Sports/Politics posts whose topic row failed to load still ranked on Explore.

This is not #122 (Phoenix request-side bitmap). Same class, later gate.

Change

• None = fetch/decode miss → drop on bulk Explore (fail closed).
• Some([]) = successful lookup, no topics → still allowed as untagged filler.
• Successful hydrator decodes now write Some([]) so the two cases stay distinct.

Proof

1. is_bulk_topic_request is topic_ids.len() > 6 (Explore multi-topic page).
2. Hydrator StratoResult::Err / empty bytes left filtered_topic_ids = None.
3. Bulk match arm _ => true kept that candidate.
4. TopicIdsFilter is pre-rank; VF is post-selection — withheld topics occupied Explore slots.
5. After: None => false, Some([]) => true; tests test_bulk_explore_drops_hydration_miss_not_known_untagged and updated test_bulk_filter_removes_only_excluded_topics.

Survey

• Pitchfork-and-Torch/x-algorithm open PRs: none (fork Why isn’t my post reaching her timeline? 😭 #41 opened by tool default and closed immediately).
• Fork PR: none.
• Distinct from upstream Stop topic retrieval from fail-opening on Sports, Science, and Technology #122 (retrieval bitmap on Sports/Science/Technology).

Base: xai-org/x-algorithm main. Head: Pitchfork-and-Torch/cursor/explore-topic-hydration-fail-open-9d2a.