Grilling: gallery block schema and editing UX

[MrTango]

Part of #23

Question

Design the gallery block's schema and editing UX so one block-data shape serves Aurora (React view) and Blicca (server renderer): which fields (image list via object browser? picture variant choice? row height / flexbin tuning? captions on/off? lightbox on/off?); how the listing gallery variation shares the same display settings; what lives in block data vs registry/control panel.

Resolution = the block schema spec.

[MrTango]

Now unblocked: #26 closed. Key input from its resolution: Aurora currently has no working listing-block variations mechanism (dead typing only; listing block being rewritten, plone/aurora#107 / #40). So the schema design here should treat the listing gallery variation as (a) firm for Blicca server-side rendering, but (b) for Aurora either deferred until the listing rewrite lands or handled by shipping our own block — decide which in this ticket. Details in docs/research/aurora-listing-variation.md (local branch research/aurora-listing-variation).