perf(explore): instant tag-cloud cache hits + fix stale-on-switch loading

The get_tag_cloud cache key was built by loading and hashing every embedding
blob for the scope *before* checking the cache, so even a cache hit re-read
hundreds of MB on large libraries and stalled Explore for several seconds.
Validate the cache from a lightweight image-ID-set signature plus the embedding
revision instead, so a hit never loads embeddings. The ID-set hash keeps the key
membership-sensitive (add/remove/move between folders) and the revision covers
an image being re-embedded in place. Cache write failures are now logged rather
than silently ignored.

On the frontend, switching folders (or re-entering Explore) no longer leaves the
previous folder's clusters/tags on screen with no loading indicator:
loadTagCloud/loadExploreTags clear stale entries on a real folder switch. The
displayed folder is tracked separately (exploreTagsShownFolderId) from the
cache-dirty marker so a same-folder invalidation (tag edits, new AI tags) does
not masquerade as a switch and wipe the visible list mid-refresh.

CHANGELOG.md

@@ -59,6 +59,10 @@ aims to follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 
 ### Changed
 
+- **Faster Explore on large libraries** — revisiting a folder's visual clusters
+  is now near-instant. The cluster cache is validated from a lightweight
+  image-ID signature instead of re-reading every embedding, so big libraries no
+  longer stall for several seconds even when the cached result is reused.
 - **Tag manager search and sort** — the Manage mode tag list now has a live
   filter input and a sort dropdown (most-used / least-used / A–Z / Z–A). The
   list is virtualised so libraries with thousands of tags scroll without lag,
@@ -86,6 +90,10 @@ aims to follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 
 ### Fixed
 
+- **Stale Explore results on folder switch** — switching folders (or re-entering
+  Explore) no longer briefly shows the previous folder's clusters/tags with no
+  loading indicator; the view now clears and shows a loading state until the new
+  folder's data arrives.
 - **Rating no longer scrambles search results** — rating or favoriting an image
   while viewing similar-image, region, semantic, tag, or album results no longer
   re-sorts the view back into the default order; the current result ordering is