Add Mixture-of-Agents (MoA) mode on top of Fusion

Extend the panel engine with a layer loop: Fusion is one proposer round,
MoA runs N rounds where each round after the first sees the previous
round's responses via <previous_layer_responses> and refines its answer.
Only the final round is persisted; intermediate rounds are live-only.
Rounds are clamped to 1-5.

- SessionRecord + migration: moa_layers column (default 2)
- RunPanel: layer loop with per-member cloned message injection
- CloneForMember / BuildPreviousLayerBlock helpers for member isolation
- SSE: panel_layer_start / panel_layer_done events
- Frontend: layer_index composite keying, round dividers, MoA option
- Docs: ARCHITECTURE + ARCHITECTURE_QUICK updated

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/js/chat.js

@@ -128,11 +128,19 @@ function handleStreamEvent(eventType, data) {
       finalizeReviewBubble('done', data, 'finalizer');
       break;
 
-    // Multi-Modell-Panel (Fusion): mehrere Member streamen parallel LIVE
+    // Multi-Modell-Panel (Fusion/MoA): mehrere Member streamen parallel LIVE
     case 'panel_start':
       startPanel(data);
       break;
 
+    // MoA: Runden-Trenner (nur bei mehreren Runden sichtbar)
+    case 'panel_layer_start':
+      startPanelLayer(data);
+      break;
+
+    case 'panel_layer_done':
+      break;
+
     case 'panel_member_start':
       addPanelMemberBubble(data);
       break;