feat(stage3): scaffold task-001 baseline

- WHY: establish Stage 3 task-001 execution baseline per repo intent
- WHAT: add minimal solution/project skeleton and boundary docs
- RULE: apply stage3 execution runtime and repository workflow directives

BuildingBlock.Inventory.slnx

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+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/BuildingBlock.Inventory.Contracts/BuildingBlock.Inventory.Contracts.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/BuildingBlock.Inventory.Contracts.UnitTests/BuildingBlock.Inventory.Contracts.UnitTests.csproj" />
+  </Folder>
+</Solution>

docs/architecture/inventory-contract-boundary.puml

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+@startuml
+skinparam packageStyle rectangle
+
+package "building-block-inventory" {
+  package "BuildingBlock.Inventory.Contracts" {
+    class Requests
+    class Responses
+    interface Abstractions
+  }
+}
+
+package "furniture-service" as FurnitureService
+package "furniture-bff" as FurnitureBff
+
+FurnitureService --> Requests
+FurnitureService --> Responses
+FurnitureBff --> Requests
+FurnitureBff --> Responses
+@enduml

docs/contracts/inventory-contract-catalog.md

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+# Inventory Contract Catalog
+
+## Package
+
+- `BuildingBlock.Inventory.Contracts`
+
+## Current Contract Groups
+
+- `Requests`: capability request shapes for inventory use cases.
+- `Responses`: capability response shapes for inventory use cases.
+- `Abstractions`: marker abstractions for inventory contract ownership.
+
+## Ownership Boundary
+
+- This repository owns inventory capability contracts only.
+- Implementation details stay in furniture-service, furniture-bff, or furniture-dal.
+- Identity abstractions are out of scope and remain Thalos-owned.

docs/contracts/inventory-versioning-policy.md

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+# Inventory Versioning Policy
+
+## Policy
+
+- Contract changes follow additive-first evolution.
+- Breaking changes require a new major package version.
+- Deprecated members should remain for at least one release cycle.
+
+## Compatibility Notes
+
+- Consumer repositories (`furniture-service`, `furniture-bff`) should update contracts explicitly.
+- Transport adapters remain outside this contracts package.

src/BuildingBlock.Inventory.Contracts/Abstractions/IInventoryCapabilityContract.cs

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+namespace BuildingBlock.Inventory.Contracts.Abstractions;
+
+/// <summary>
+/// Marker contract for inventory capability request/response definitions.
+/// </summary>
+public interface IInventoryCapabilityContract
+{
+}

src/BuildingBlock.Inventory.Contracts/BuildingBlock.Inventory.Contracts.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+</Project>

src/BuildingBlock.Inventory.Contracts/Requests/InventoryItemLookupRequest.cs

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+namespace BuildingBlock.Inventory.Contracts.Requests;
+
+/// <summary>
+/// Represents the contract request to lookup an inventory item.
+/// </summary>
+/// <param name="ItemCode">External item identifier.</param>
+public sealed record InventoryItemLookupRequest(string ItemCode);

src/BuildingBlock.Inventory.Contracts/Responses/InventoryItemLookupResponse.cs

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+namespace BuildingBlock.Inventory.Contracts.Responses;
+
+/// <summary>
+/// Represents the contract response for an inventory item lookup.
+/// </summary>
+/// <param name="ItemCode">External item identifier.</param>
+/// <param name="QuantityAvailable">Current quantity available.</param>
+public sealed record InventoryItemLookupResponse(string ItemCode, int QuantityAvailable);

tests/BuildingBlock.Inventory.Contracts.UnitTests/BuildingBlock.Inventory.Contracts.UnitTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\BuildingBlock.Inventory.Contracts\BuildingBlock.Inventory.Contracts.csproj" />
+  </ItemGroup>
+</Project>

tests/BuildingBlock.Inventory.Contracts.UnitTests/ContractShapeTests.cs

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+using BuildingBlock.Inventory.Contracts.Requests;
+using BuildingBlock.Inventory.Contracts.Responses;
+
+namespace BuildingBlock.Inventory.Contracts.UnitTests;
+
+public class ContractShapeTests
+{
+    [Fact]
+    public void InventoryItemLookupRequest_WhenCreated_StoresItemCode()
+    {
+        var request = new InventoryItemLookupRequest("SKU-001");
+
+        Assert.Equal("SKU-001", request.ItemCode);
+    }
+
+    [Fact]
+    public void InventoryItemLookupResponse_WhenCreated_StoresContractData()
+    {
+        var response = new InventoryItemLookupResponse("SKU-001", 5);
+
+        Assert.Equal("SKU-001", response.ItemCode);
+        Assert.Equal(5, response.QuantityAvailable);
+    }
+}