core: split message part updates into delta events for smoother streaming

Streaming text and reasoning content now uses incremental delta events instead of sending full message parts on each update. This reduces bandwidth and improves real-time response smoothness in the TUI.

packages/app/src/context/global-sync.tsx

@@ -839,6 +839,23 @@ function createGlobalSync() {
         )
         break
       }
+      case "message.part.delta": {
+        const parts = store.part[event.properties.messageID]
+        if (!parts) break
+        const result = Binary.search(parts, event.properties.partID, (p) => p.id)
+        if (!result.found) break
+        setStore(
+          "part",
+          event.properties.messageID,
+          produce((draft) => {
+            const part = draft[result.index]
+            const field = event.properties.field as keyof typeof part
+            const existing = part[field] as string | undefined
+            ;(part[field] as string) = (existing ?? "") + event.properties.delta
+          }),
+        )
+        break
+      }
       case "message.part.removed": {
         const messageID = event.properties.messageID
         const parts = store.part[messageID]