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/components/Post/Embed/ImageEmbed.tsx

@@ -1,4 +1,4 @@
-import {useRef} from 'react'
+import {useEffect, useRef} from 'react'
 import {InteractionManager, View} from 'react-native'
 import {type AnimatedRef} from 'react-native-reanimated'
 import {Image} from 'expo-image'
@@ -8,7 +8,10 @@ import {atoms as a, tokens} from '#/alf'
 import {AutoSizedImage} from '#/components/images/AutoSizedImage'
 import {Gallery} from '#/components/images/Gallery'
 import {ImageLayoutGrid} from '#/components/images/ImageLayoutGrid'
-import {useLightboxControls} from '#/components/Lightbox/state'
+import {
+  type LightboxMetricsContext,
+  useLightboxControls,
+} from '#/components/Lightbox/state'
 import {type Dimensions} from '#/components/Lightbox/types'
 import {ImageContextMenu} from '#/components/Post/Embed/ImageContextMenu'
 import {PostEmbedViewContext} from '#/components/Post/Embed/types'
@@ -40,6 +43,35 @@ export function ImageEmbed({
       ? images.length > MAX_GRID_IMAGES
       : ax.features.enabled(ax.features.PostGalleryEmbedEnable)
 
+  const layout: 'single' | 'grid' | 'carousel' =
+    images.length === 1 ? 'single' : useExpandedLayout ? 'carousel' : 'grid'
+
+  const postContext =
+    rest.uri && rest.authorDid
+      ? {
+          uri: rest.uri,
+          authorDid: rest.authorDid,
+          feedDescriptor: rest.feedDescriptor,
+        }
+      : undefined
+  const metricsContext: LightboxMetricsContext | undefined = postContext
+    ? {layout, ...postContext}
+    : undefined
+
+  // Impression: one per mount of a post photo embed. Covers all three layouts
+  // identically so the opens/impressions CTR is unbiased by layout.
+  useEffect(() => {
+    if (images.length > 0 && postContext) {
+      ax.metric('post:photoEmbed:impression', {
+        layout,
+        totalImages: images.length,
+        ...postContext,
+      })
+    }
+    // Fire once per mount; intentionally not reactive to post context changes.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [])
+
   // Captured from AutoSizedImage so the peek-commit handler can reuse the same
   // ref + dims that a tap would — keeps the lightbox's return animation intact.
   const singleContainerRef = useRef<AnimatedRef<any> | null>(null)
@@ -57,6 +89,14 @@ export function ImageEmbed({
       refs: AnimatedRef<any>[],
       fetchedDims: (Dimensions | null)[],
     ) => {
+      if (postContext) {
+        ax.metric('post:photoEmbed:open', {
+          layout,
+          fromImage: index + 1,
+          totalImages: images.length,
+          ...postContext,
+        })
+      }
       openLightbox({
         images: items.map((item, i) => ({
           ...item,
@@ -67,6 +107,7 @@ export function ImageEmbed({
           type: 'image',
         })),
         index,
+        metricsContext,
       })
     }
     const onPressIn = (_: number) => {
@@ -132,6 +173,7 @@ export function ImageEmbed({
             onPressIn={onPressIn}
             viewContext={rest.viewContext}
             isWithinQuote={rest.isWithinQuote}
+            metricsPostContext={postContext}
           />
         </View>
       )

src/components/Post/Embed/index.tsx

@@ -345,6 +345,10 @@ export function QuoteEmbed({
           allowNestedQuotes={
             parentIsWithinQuote ? false : parentAllowNestedQuotes
           }
+          // The photo embed belongs to the quoted post, so attribute its
+          // analytics to the quoted post rather than the parent.
+          uri={quote.uri}
+          authorDid={quote.author.did}
         />
       )}
     </>

src/components/Post/Embed/types.ts

@@ -15,6 +15,12 @@ export type CommonProps = {
   viewContext?: PostEmbedViewContext
   isWithinQuote?: boolean
   allowNestedQuotes?: boolean
+  // Post context for analytics on photo embed events (post:photoEmbed:*).
+  // When the embed has no owning post (e.g. previews), leave these undefined
+  // and no events will be emitted.
+  uri?: string
+  authorDid?: string
+  feedDescriptor?: string
 }
 
 export type EmbedProps = CommonProps & {