Rebuild: an image without words around it is keyed as a stem

So a cover 'x.jpg' joins 'x.part01.rar' in a stem-named post; an image
set like 'IMG_008' keys as the stem 'IMG'. Both implementations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/release/identity.ts

@@ -226,8 +226,8 @@ export function releaseIdentity(input: ReleaseInput): ReleaseIdentity | null {
   if (!tag && fileTotal && isPostToken(remainder)) {
     return { key: hash('post', input.poster, remainder, fileTotal), name: remainder, fileTotal, source: 'post' }
   }
-  // An image without words around it: its set by its name.
-  if (image) return { key: hash('images', input.poster, prefix), name: prefix || stem, fileTotal, source: 'stem' }
+  // An image without words around it: its set by its name (keyed as a stem, so "x.jpg" joins "x.part01.rar").
+  if (image) return { key: hash('stem', input.poster, prefix || stem), name: prefix || stem, fileTotal, source: 'stem' }
   // The [i/M] total is informational, not part of the key: posters often
   // upload the par2 index without a counter, and it belongs to the same post.
   return { key: hash('stem', input.poster, stem), name: stem, fileTotal, source: 'stem' }