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/components/chat/ModelControls.tsx

@@ -63,6 +63,7 @@ import { useSessionUIStore } from '@/sync/session-ui-store';
 import { useSelectionStore } from '@/sync/selection-store';
 import { useDirectorySync, useSessionMessages } from '@/sync/sync-context';
 import { useSync } from '@/sync/use-sync';
+import { getSessionMaterializationStatus } from '@/sync/materialization';
 import { useUIStore } from '@/stores/useUIStore';
 import { useModelLists } from '@/hooks/useModelLists';
 import { useIsTextTruncated } from '@/hooks/useIsTextTruncated';
@@ -768,9 +769,9 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
     const latestLoadedUserChoiceRestoreRef = React.useRef<string | null>(null);
 
     const currentSessionDirectory = currentSessionId ? getDirectoryForSession(currentSessionId) : undefined;
-    const hasCurrentSessionMessagesEntry = useDirectorySync(
+    const hasRenderableCurrentSessionSnapshot = useDirectorySync(
         React.useCallback(
-            (state) => (currentSessionId ? state.message[currentSessionId] !== undefined : false),
+            (state) => (currentSessionId ? getSessionMaterializationStatus(state, currentSessionId).renderable : false),
             [currentSessionId],
         ),
         currentSessionDirectory ?? undefined,
@@ -942,7 +943,7 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
             return;
         }
 
-        if (!contextHydrated || providers.length === 0 || !hasCurrentSessionMessagesEntry || !latestLoadedUserChoice?.providerID || !latestLoadedUserChoice.modelID) {
+        if (!contextHydrated || providers.length === 0 || !hasRenderableCurrentSessionSnapshot || !latestLoadedUserChoice?.providerID || !latestLoadedUserChoice.modelID) {
             return;
         }
 
@@ -990,7 +991,7 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
         currentAgentName,
         contextHydrated,
         providers,
-        hasCurrentSessionMessagesEntry,
+        hasRenderableCurrentSessionSnapshot,
         latestLoadedUserChoice,
         setAgent,
         tryApplyModelSelection,
@@ -1113,9 +1114,9 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
             return;
         }
 
-        if (!hasCurrentSessionMessagesEntry) {
+        if (!hasRenderableCurrentSessionSnapshot) {
             if (!sync.isLoading(currentSessionId)) {
-                void sync.syncSession(currentSessionId);
+                void sync.ensureSessionRenderable(currentSessionId);
             }
             return;
         }
@@ -1127,7 +1128,7 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
         applyFallbackAgent();
     }, [
         currentSessionId,
-        hasCurrentSessionMessagesEntry,
+        hasRenderableCurrentSessionSnapshot,
         latestLoadedUserChoice,
         agents,
         primaryAgents,