Drop quotes, retweets, and replies whose ancillary VF verdict is Interstitial

[Pitchfork-and-Torch]

Bug

#134 leftover. Quote / RT / ancestor Action::Interstitial does not set drop_ancillary_posts. Verified on xai-org/main (902a06f).

should_drop_reason only matched Action::Drop. Home NSFW / gore / card / nsfw-author rules emit Interstitial. Recs has Drop twins, but RT sources are fetched at TimelineHome, and a dual-role id is overwritten by the Home Interstitial. AncillaryVFFilter reads the flag. The wrapper visibility_reason is the primary Allow. Mixer serializes only that. This repo does not draw an interstitial.

This is not #115 (Following quotes at Home vs Recs). Not #119 (RT primary looked up by wrapper id). Not #121 (VF Err / missing key). #121 leftover text called this Interstitial≠Drop path out. #134 left it as keep-and-warn. Keep-and-warn is only real on the primary card.

Five-line proof

• Entry: VFCandidateHydrator::should_drop_reason (quote / RT / ancestor lookups in should_drop_ancillary)
• Sink: AncillaryVFFilter (drop_ancillary_posts); candidates_to_scored_posts writes only primary visibility_reason
• Break: Action::Interstitial on a child did not set the flag; wrapper stayed Allow
• Viewer effect: a followee RT (or quote/reply) of NSFW / gore serves as a normal For You / Following card with the media embed and no warn
• Twin: Recs Drop twins already drop the same labels when the child is fetched at TimelineHomeRecommendations. Primary Interstitial still keep-and-warn via VFFilter / visibility_reason

Change

Treat Action::Interstitial like Action::Drop for ancillary children only. Primary Interstitial is unchanged.

Tests

• Quote / RT / ancestor Interstitial → drop_ancillary_posts
• Tombstoned ancestor Interstitial still skipped
• Ancillary Allow keeps; ancillary Drop still drops
• Primary Interstitial with no child does not set the flag

Standalone decision-table harness (same match arms): 9/9 passed.

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

Fork PR: none