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/css/chat.css

@@ -1227,6 +1227,34 @@
   border-color: var(--accent, #9b6dff);
 }
 
+/* MoA: Trenner zwischen Proposer-Runden */
+.panel-layer-divider {
+  display: flex;
+  align-items: center;
+  gap: .6em;
+  margin: 1.1em 0 .5em;
+  color: var(--accent, #9b6dff);
+  font-size: .82em;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: .04em;
+  opacity: .85;
+}
+.panel-layer-divider::before,
+.panel-layer-divider::after {
+  content: "";
+  flex: 1;
+  height: 1px;
+  background: var(--accent, #9b6dff);
+  opacity: .35;
+}
+.panel-layer-divider span {
+  display: inline-flex;
+  align-items: center;
+  gap: .4em;
+  white-space: nowrap;
+}
+
 .review-inline-feedback {
   color: var(--warning);
   margin-bottom: .4em;