feat(furniture-bff): enforce single active edge protocol

docs/architecture/bff-service-boundary.md

@@ -7,6 +7,7 @@ Preserve BFF as an edge adapter layer that depends on service contracts only.
 - REST edge exposure
 - Service client adaptation
 - Correlation/tracing propagation
+- Single active edge protocol policy enforcement (`rest`)
 
 ## Prohibited
 - Direct DAL access

src/Furniture.Bff.Rest/Program.cs

@@ -11,6 +11,12 @@ using Microsoft.Extensions.Primitives;
 const string CorrelationHeaderName = "x-correlation-id";
 
 var builder = WebApplication.CreateBuilder(args);
+var edgeProtocol = builder.Configuration["FurnitureBff:EdgeProtocol"] ?? "rest";
+if (!string.Equals(edgeProtocol, "rest", StringComparison.OrdinalIgnoreCase))
+{
+    throw new InvalidOperationException(
+        $"Furniture BFF supports one active edge protocol per deployment. Configured: '{edgeProtocol}'. Expected: 'rest'.");
+}
 
 builder.Services.AddHttpContextAccessor();
 builder.Services.AddHealthChecks();