fix: await concurrent attachment uploads

.project-docs/30-worklog/tasks/20260823-pi-chat-timeline-c8f31a62.md

@@ -156,7 +156,8 @@
   acceptance settle, so removal or a next-message paste cannot mutate the
   captured image set. Attachment preparation failures now remain local to the
   Composer, restore the draft, permit direct retry without runtime recovery,
-  and retain already uploaded variants for reuse. Attachment route failures use
+  retain already uploaded variants for reuse, and wait for every concurrent
+  upload flight to settle before releasing the edit lock. Attachment route failures use
   only registered `CODING_ATTACHMENT_INVALID`, `CODING_ATTACHMENT_NOT_FOUND`,
   and `CODING_STORAGE_WRITE_FAILED` codes while preserving 413/404 status.
 - Synchronized `README.md` with the current Pi core Chat, first-Conversation,