Replaces the legacy post:gallery:* events, which only instrumented the
carousel layout (and so were biased by the PostGalleryEmbedEnable rollout)
with a coherent, layout-agnostic family that fires identically across the
single image, 2-4 image grid, and carousel renderers.
The new events all carry uri, authorDid, and an optional feedDescriptor,
and a layout discriminator ('single' | 'grid' | 'carousel') so opens and
impressions are directly comparable across layouts.
- post:photoEmbed:impression: fires once per mount of a post photo embed
- post:photoEmbed:open: fires from the shared onPress in ImageEmbed, so
all three layouts emit it without per-layout duplication
- post:photoEmbed:carouselSwipe: in-feed carousel swipe (debounced)
- post:photoEmbed:lightboxSwipe: fires from the lightbox pager regardless
of which layout opened it
The dead post:gallery:impression definition is removed, and
post:gallery:openLightbox / post:gallery:swipe are hard-removed with no
dual-emit period.
Quoted-post photos are attributed to the quoted post (quote.uri /
quote.author.did), not the surrounding post.
vineyardbovines
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