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/pages/session-page.js

@@ -33,6 +33,8 @@ registerPage('session', {
     const panelAgentsRow = document.getElementById('panelAgentsRow');
     const panelAgentsCard = document.getElementById('panelAgentsCard');
     const panelAgentIdsEl = document.getElementById('panelAgentIds');
+    const moaLayersRow = document.getElementById('moaLayersRow');
+    const moaLayersEl = document.getElementById('moaLayers');
     const imagePreview = document.getElementById('sessionImagePreview');
     const saveBtn = document.getElementById('sessionSaveBtn');
     const cancelBtn = document.getElementById('sessionCancelBtn');
@@ -148,9 +150,11 @@ registerPage('session', {
     }
     function updatePanelVisibility() {
       panelAgentsRow.hidden = (panelModeEl.value === 'off');
+      if (moaLayersRow) moaLayersRow.hidden = (panelModeEl.value !== 'moa');
     }
     panelModeEl.value = (session && session.multiModelMode) || 'off';
     panelAgentIdsEl.value = (session && session.panelAgentIds) || '';
+    if (moaLayersEl) moaLayersEl.value = (session && session.moaLayers) || 2;
     renderPanelInCard();
     updatePanelVisibility();
     panelModeEl.addEventListener('change', updatePanelVisibility);
@@ -215,6 +219,7 @@ registerPage('session', {
         preparerAgentId: preparerAgentSelect.value || null,
         multiModelMode: panelModeEl.value || 'off',
         panelAgentIds: panelAgentIdsEl.value || null,
+        moaLayers: moaLayersEl ? (parseInt(moaLayersEl.value, 10) || 2) : 2,
         context: _sessionContextMde.value().trim() || null,
         workspace: workspaceEl.value.trim() || null,
         customCommands: commands,