feat(furniture-domain): enforce availability decision invariants

docs/migration/behavior-invariants.md

@@ -4,6 +4,8 @@
 - Availability decision outcome remains unchanged for equivalent inputs.
 - Correlation propagation behavior remains unchanged.
 - Transport contracts stay stable at service boundary.
+- Missing display names map to `Unknown Furniture`.
+- Negative inventory quantities are clamped to `0`.
 
 ## Validation Approach
 - Compare pre/post extraction contract examples.

src/Furniture.Domain/Decisions/FurnitureAvailabilityDecisionService.cs

@@ -38,10 +38,17 @@ public sealed class FurnitureAvailabilityDecisionService : IFurnitureAvailabilit
         ProductContractResponse catalogResponse,
         InventoryItemLookupResponse inventoryResponse)
     {
+        var displayName = string.IsNullOrWhiteSpace(catalogResponse.DisplayName)
+            ? "Unknown Furniture"
+            : catalogResponse.DisplayName;
+        var quantityAvailable = inventoryResponse.QuantityAvailable < 0
+            ? 0
+            : inventoryResponse.QuantityAvailable;
+
         return new FurnitureAvailabilityDecisionResponse(
             request.FurnitureId,
-            catalogResponse.DisplayName,
-            inventoryResponse.QuantityAvailable);
+            displayName,
+            quantityAvailable);
     }
 
     private static string ResolveCorrelationId(string correlationId)

tests/Furniture.Domain.UnitTests/FurnitureAvailabilityDecisionServiceTests.cs

@@ -51,4 +51,42 @@ public class FurnitureAvailabilityDecisionServiceTests
         Assert.Equal("Chair", response.DisplayName);
         Assert.Equal(12, response.QuantityAvailable);
     }
+
+    [Fact]
+    public void ComposeResponse_WhenCatalogDisplayNameMissing_UsesFallbackName()
+    {
+        var service = new FurnitureAvailabilityDecisionService();
+        var request = new FurnitureAvailabilityDecisionRequest("FUR-004", "corr-004");
+        var catalog = new ProductContractResponse(
+            new CatalogContractEnvelope("1.0.0", "corr-004"),
+            "FUR-004",
+            string.Empty);
+        var inventory = new InventoryItemLookupResponse(
+            new InventoryContractEnvelope("1.0.0", "corr-004"),
+            "FUR-004",
+            5);
+
+        var response = service.ComposeResponse(request, catalog, inventory);
+
+        Assert.Equal("Unknown Furniture", response.DisplayName);
+    }
+
+    [Fact]
+    public void ComposeResponse_WhenInventoryNegative_ClampsQuantityToZero()
+    {
+        var service = new FurnitureAvailabilityDecisionService();
+        var request = new FurnitureAvailabilityDecisionRequest("FUR-005", "corr-005");
+        var catalog = new ProductContractResponse(
+            new CatalogContractEnvelope("1.0.0", "corr-005"),
+            "FUR-005",
+            "Shelf");
+        var inventory = new InventoryItemLookupResponse(
+            new InventoryContractEnvelope("1.0.0", "corr-005"),
+            "FUR-005",
+            -3);
+
+        var response = service.ComposeResponse(request, catalog, inventory);
+
+        Assert.Equal(0, response.QuantityAvailable);
+    }
 }