Wires the new gallery embed (atproto PR #4827) through the display and
compose pipelines while the lexicon is unpublished. Gallery posts render
via the existing carousel for >4 items and grid for <=4. Compose now
allows up to 10 images and auto-promotes images -> gallery above 4
(demotes back when count drops to <=4 so old clients still see legacy
embeds when possible). Drafts persist as the new `embedGallery` field.
Local shim types in `src/lib/api/gallery-embed-shim*.ts` mirror the
lexicon shape; delete both files once @atproto/api ships the generated
types.
vineyardbovines
9aea4362d5