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>

Models/SessionRecord.cs

@@ -23,8 +23,9 @@ public class SessionRecord
     public int MaxReviewIterations { get; set; } = 3;
     public string FinalizerAgentId { get; set; }
     public string PreparerAgentId { get; set; }
-    public string MultiModelMode { get; set; }   // "off" / "fusion" (später "moa")
+    public string MultiModelMode { get; set; }   // "off" / "fusion" / "moa"
     public string PanelAgentIds { get; set; }     // CSV der Panel-Agenten (Proposer)
+    public int MoaLayers { get; set; } = 2;       // MoA: Anzahl Proposer-Runden vor der Synthese (Fusion ignoriert)
     public string UnityMcpUrl { get; set; }
     public string UnityMcpToken { get; set; }
     public string CreatedAt { get; set; }