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.Catalog.slnx

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

docs/architecture/catalog-contract-boundary.puml

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+@startuml
+skinparam packageStyle rectangle
+
+package "building-block-catalog" {
+  package "BuildingBlock.Catalog.Contracts" {
+    class Products
+    class Tags
+    interface Abstractions
+  }
+}
+
+package "furniture-dal" as FurnitureDal
+package "furniture-service" as FurnitureService
+package "furniture-bff" as FurnitureBff
+
+FurnitureDal --> Products
+FurnitureDal --> Tags
+FurnitureService --> Products
+FurnitureService --> Tags
+FurnitureBff --> Products
+FurnitureBff --> Tags
+@enduml

docs/contracts/catalog-contract-catalog.md

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+# Catalog Contract Catalog
+
+## Package
+
+- `BuildingBlock.Catalog.Contracts`
+
+## Contract Groups
+
+- `Products`: product contract shapes.
+- `Tags`: tag, tag type, and tag override contract shapes.
+- `Abstractions`: marker abstraction for contract ownership.
+
+## Ownership Boundary
+
+- This repository owns reusable catalog capability contracts.
+- Persistence and transport concerns remain outside this package.
+- Identity abstractions remain Thalos-owned.

docs/contracts/catalog-versioning-policy.md

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+# Catalog Versioning Policy
+
+## Policy
+
+- Prefer additive contract evolution to keep consumer compatibility.
+- Breaking changes require major version increments.
+- Deprecated members remain through one deprecation cycle.
+
+## Compatibility Notes
+
+- Consumers (`furniture-dal`, `furniture-service`, `furniture-bff`) update explicitly.
+- Transport-specific adapters are out of scope for this contracts package.

src/BuildingBlock.Catalog.Contracts/Abstractions/ICatalogCapabilityContract.cs

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+namespace BuildingBlock.Catalog.Contracts.Abstractions;
+
+/// <summary>
+/// Marker contract for catalog capability request/response definitions.
+/// </summary>
+public interface ICatalogCapabilityContract
+{
+}

src/BuildingBlock.Catalog.Contracts/BuildingBlock.Catalog.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.Catalog.Contracts/Products/ProductContract.cs

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+namespace BuildingBlock.Catalog.Contracts.Products;
+
+/// <summary>
+/// Catalog product contract.
+/// </summary>
+/// <param name="ProductId">Product identifier in catalog capability scope.</param>
+/// <param name="DisplayName">Product display name.</param>
+public sealed record ProductContract(string ProductId, string DisplayName);

src/BuildingBlock.Catalog.Contracts/Tags/TagContract.cs

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+namespace BuildingBlock.Catalog.Contracts.Tags;
+
+/// <summary>
+/// Catalog tag contract.
+/// </summary>
+/// <param name="TagId">Tag identifier.</param>
+/// <param name="TagTypeId">Tag type identifier.</param>
+/// <param name="Value">Tag value.</param>
+public sealed record TagContract(string TagId, string TagTypeId, string Value);

src/BuildingBlock.Catalog.Contracts/Tags/TagOverrideContract.cs

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+namespace BuildingBlock.Catalog.Contracts.Tags;
+
+/// <summary>
+/// Catalog tag override contract for consumer-specific overrides.
+/// </summary>
+/// <param name="TagId">Tag identifier.</param>
+/// <param name="TargetScope">Override target scope.</param>
+/// <param name="OverrideValue">Override value.</param>
+public sealed record TagOverrideContract(string TagId, string TargetScope, string OverrideValue);

src/BuildingBlock.Catalog.Contracts/Tags/TagTypeContract.cs

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+namespace BuildingBlock.Catalog.Contracts.Tags;
+
+/// <summary>
+/// Catalog tag type contract.
+/// </summary>
+/// <param name="TagTypeId">Tag type identifier.</param>
+/// <param name="Name">Tag type name.</param>
+public sealed record TagTypeContract(string TagTypeId, string Name);

tests/BuildingBlock.Catalog.Contracts.UnitTests/BuildingBlock.Catalog.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.Catalog.Contracts\BuildingBlock.Catalog.Contracts.csproj" />
+  </ItemGroup>
+</Project>

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

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+using BuildingBlock.Catalog.Contracts.Products;
+using BuildingBlock.Catalog.Contracts.Tags;
+
+namespace BuildingBlock.Catalog.Contracts.UnitTests;
+
+public class ContractShapeTests
+{
+    [Fact]
+    public void ProductContract_WhenCreated_StoresRequiredValues()
+    {
+        var contract = new ProductContract("PRD-001", "Chair");
+
+        Assert.Equal("PRD-001", contract.ProductId);
+        Assert.Equal("Chair", contract.DisplayName);
+    }
+
+    [Fact]
+    public void TagOverrideContract_WhenCreated_StoresRequiredValues()
+    {
+        var contract = new TagOverrideContract("TAG-001", "furniture", "featured");
+
+        Assert.Equal("TAG-001", contract.TagId);
+        Assert.Equal("furniture", contract.TargetScope);
+        Assert.Equal("featured", contract.OverrideValue);
+    }
+}