Fix event-variant filter false-positive: require the share target to exist

[cubehouse]

Problem

The event-variant filter added in the prior USH fix (isEventVariantAlias) dropped any is_event=true place whose social_sharing_link?id= differed from its own place_id. That's too broad: it also drops legit entities whose share link points at an id that doesn't exist in the feed.

Concrete case — UOR Epic Universe: uor.ueu.show.meet_donkey_kong (and Meet Toad, The Cosmos Fountain) are real standalone shows, but their share link points at a phantom uor.ueu.entertainment.meet_donkey_kong the feed never emits. They were being dropped → and since they're live on the site, the sync marked them for deletion.

Fix

A place is a variant only when its share target is a different place that actually exists in the feed (a real canonical to defer to). The check now takes the set of real place_ids and moved from placeToEntity into buildEntityList (which has the full place list). Self-links and links to phantom ids are kept.

Validation (live feed, both impacted parks — UOR + USH are the only classes using this code)

• UOR: Meet Donkey Kong / Meet Toad / The Cosmos Fountain now kept (were wrongly deleted). Still drops Hogwarts Express first/last train (true variants → canonical hogwarts_express which exists). Remaining deletions are unrelated (Summer Music Series is_event=false; Thunder Falls Terrace gone from feed).
• USH: unchanged — Studio Tour language/last-tram variants still dropped, all 41 entities resolve, 0 unresolved parents.
• Unit tests updated to exercise the existence check directly (self-link kept, phantom-target kept, real-target dropped). Full suite: 1523 pass.