refactor: stabilize live chat sync materialization (#1132)

Canonicalize session message/part materialization across load, prefetch, reconnect, and recovery paths so OpenChamber restores session snapshots through one consistent merge flow.

Preserve live assistant streaming text when stale or delayed snapshots arrive, while still replacing optimistic user parts with confirmed server snapshots to avoid duplicated user messages.

Narrow recovery triggers to explicit incomplete snapshot signals instead of broad session-event fallbacks, reducing unnecessary session refetches during active streaming.

Keep turn windowing aligned with parented assistant replies and add regression coverage for materialization gaps, stale snapshot protection, optimistic user replacement, reconnect recovery, and turn grouping.

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

@@ -101,6 +101,23 @@ export type GlobalEventResult = {
   project: Project
 } | null
 
+export type DirectoryEventResult = boolean | {
+  changed: boolean
+  materialization: {
+    type: "incomplete-session-snapshot"
+    sessionID?: string
+    messageID: string
+    partID?: string
+  }
+}
+
+function hasMessage(draft: State, sessionID: string | undefined, messageID: string): boolean {
+  if (!sessionID) return false
+  const messages = draft.message[sessionID]
+  if (!messages) return false
+  return Binary.search(messages, messageID, (message) => message.id).found
+}
+
 export function reduceGlobalEvent(event: Event): GlobalEventResult {
   if (event.type === "global.disposed" || event.type === "server.connected") {
     return { type: "refresh" }
@@ -135,7 +152,7 @@ export function applyDirectoryEvent(
     onLoadLsp?: () => void
     onSetSessionTodo?: (sessionID: string, todos: Todo[] | undefined) => void
   },
-): boolean {
+): DirectoryEventResult {
   switch (event.type) {
     case "server.instance.disposed": {
       callbacks?.onRefresh?.("")
@@ -269,11 +286,18 @@ export function applyDirectoryEvent(
         return false
       }
       const messageID = (part as { messageID: string }).messageID
+      const sessionID = (part as { sessionID?: string }).sessionID
+      const missingOwningMessage = !hasMessage(draft, sessionID, messageID)
       const parts = draft.part[messageID]
       if (!parts) {
         syncDebug.reducer.partUpdatedNoExistingParts(messageID, part.id, part.type)
         draft.part[messageID] = [part]
-        return true
+        return missingOwningMessage
+          ? {
+            changed: true,
+            materialization: { type: "incomplete-session-snapshot", sessionID, messageID, partID: part.id },
+          }
+          : true
       }
       const next = [...parts]
       const result = Binary.search(next, part.id, (p) => p.id)
@@ -302,7 +326,12 @@ export function applyDirectoryEvent(
         next.splice(insertResult.index, 0, part)
       }
       draft.part[messageID] = next
-      return true
+      return missingOwningMessage
+        ? {
+          changed: true,
+          materialization: { type: "incomplete-session-snapshot", sessionID, messageID, partID: part.id },
+        }
+        : true
     }
 
     case "message.part.removed": {
@@ -333,12 +362,18 @@ export function applyDirectoryEvent(
       const parts = draft.part[props.messageID]
       if (!parts) {
         syncDebug.reducer.partDeltaNoParts(props.messageID, props.partID)
-        return false
+        return {
+          changed: false,
+          materialization: { type: "incomplete-session-snapshot", messageID: props.messageID, partID: props.partID },
+        }
       }
       const result = Binary.search(parts, props.partID, (p) => p.id)
       if (!result.found) {
         syncDebug.reducer.partDeltaNotFound(props.messageID, props.partID)
-        return false
+        return {
+          changed: false,
+          materialization: { type: "incomplete-session-snapshot", messageID: props.messageID, partID: props.partID },
+        }
       }
       const existing = parts[result.index] as Record<string, unknown>
       const existingValue = existing[props.field] as string | undefined