fix(sync): replace every optimistic part of a just-sent message

The server echoes a sent message part by part. The reducer replaced an
optimistic part with its server echo only while the FIRST part of the
message was still optimistic, so once the text echo had landed the file
echo no longer qualified and was appended instead: an attached image
showed twice until the next page fetch rewrote the parts.

Any remaining optimistic part of the same type is now a replacement
candidate. The scan only runs when a part with a new id arrives, which is
once per part during assistant streaming.

Testing: reducer test extended past the first echo to the file echo; ui
type-check and lint; verified the event order against a live OpenCode
(message, text part, file part within ~70ms of the accepted request).

packages/ui/src/sync/__tests__/event-reducer.test.ts

@@ -128,6 +128,15 @@ describe("applyDirectoryEvent", () => {
       properties: { part: serverText },
     } as Event)).toBe(true)
     expect(draft.part.msg_1).toEqual([serverText, optimisticFile])
+
+    // The file echo follows the text echo; it must claim the optimistic file
+    // even though the first slot now holds a server part.
+    const serverFile = { id: "prt_server_file", messageID: "msg_1", sessionID: "ses_1", type: "file", filename: "a.png" } as Part
+    expect(applyDirectoryEvent(draft, {
+      type: "message.part.updated",
+      properties: { part: serverFile },
+    } as Event)).toBe(true)
+    expect(draft.part.msg_1).toEqual([serverText, serverFile])
   })
 
   test("returns typed materialization when delta arrives before parts", () => {

packages/ui/src/sync/event-reducer.ts

@@ -433,11 +433,13 @@ export function applyDirectoryEvent(
           : part
       } else {
         // Replace optimistic part (no sessionID) with server part of same type.
-        // Gate: only scan if the first part lacks sessionID (optimistic parts are
-        // always inserted first). Assistant messages never have optimistic parts,
-        // so this check is effectively free during streaming.
-        const hasOptimistic = next.length > 0 && !(next[0] as { sessionID?: string }).sessionID
-        const optimisticIndex = hasOptimistic && (part.type === "text" || part.type === "file")
+        // Every optimistic part is a candidate, not only the first one: the
+        // server echoes a just-sent message part by part, and after the text
+        // echo replaced the first slot the file echo still has to find the
+        // optimistic file behind it, or the attachment shows twice until the
+        // next page fetch. The scan runs only when a part with a NEW id
+        // arrives, which during assistant streaming is once per part.
+        const optimisticIndex = part.type === "text" || part.type === "file"
           ? next.findIndex((p) => p.type === part.type && !(p as { sessionID?: string }).sessionID)
           : -1
         if (optimisticIndex >= 0) {