feat(platform): upload activity images via Hyperdrive staging

Add setActivityImageFromBytes (256 KB image cap, __presence__ channel) and
readActivityImagePreview for UI thumbnails. Merges large/small image keys
into the current activity and reuses presence enrichment for P2P gossip.

index.js

@@ -5174,6 +5174,51 @@ class PearcordPlatform extends EventEmitter {
     return base
   }
 
+  async setActivityImageFromBytes ({ data, filename, mimeType, slot = 'large' }) {
+    if (!this.attachments) throw new Error('attachments not ready')
+    const user = this.identity.user
+    if (!user) throw new Error('register first')
+    const mt = String(mimeType || '')
+    if (!mt.startsWith('image/')) throw new Error('activity image must be image/*')
+    if (data && data.length > 256 * 1024) {
+      throw new Error('activity image must be 256 KB or smaller')
+    }
+    const guildId = this.guild?.guild?.id || '__presence__'
+    const att = await this.attachments.stage({
+      data,
+      filename: filename || 'activity.png',
+      mimeType: mt,
+      guildId,
+      channelId: '__presence__',
+      authorId: user.id
+    })
+    if (this.guild) this.guild.gossipAttachmentMeta(att)
+    const cur = this.presence?.activity || {
+      type: 'custom',
+      name: 'Activity',
+      startedAt: Date.now()
+    }
+    const assets = { ...(cur.assets || {}) }
+    if (String(slot).toLowerCase() === 'small') {
+      assets.smallImageKey = att.id
+    } else {
+      assets.largeImageKey = att.id
+    }
+    return this.setActivity({ ...cur, assets })
+  }
+
+  async readActivityImagePreview (attachmentId, slot = 'large') {
+    const id =
+      attachmentId ||
+      (String(slot).toLowerCase() === 'small'
+        ? this.presence?.activity?.assets?.smallImageKey
+        : this.presence?.activity?.assets?.largeImageKey) ||
+      this.presence?.activity?.assets?.largeImageKey
+    if (!id) return null
+    await this._ensureBannerAttachmentReady(id)
+    return this.readAttachmentPreview(id)
+  }
+
   setActivity (activity) {
     if (!this.presence) throw new Error('presence unavailable')
     return this.setPresence(