Refactor photo embed analytics to post:photoEmbed:* namespace

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.

src/screens/PostThread/components/ThreadItemAnchor.tsx

@@ -414,6 +414,8 @@ const ThreadItemAnchorInner = memo(function ThreadItemAnchorInner({
                     moderation={moderation}
                     viewContext={PostEmbedViewContext.ThreadHighlighted}
                     onOpen={onOpenEmbed}
+                    uri={post.uri}
+                    authorDid={post.author.did}
                   />
                 </View>
               )}