Complete batch 012 (golden set diff) and 013 (advisory chat), fix build errors

Sprints completed:
- SPRINT_20260110_012_* (golden set diff layer - 10 sprints)
- SPRINT_20260110_013_* (advisory chat - 4 sprints)

Build fixes applied:
- Fix namespace conflicts with Microsoft.Extensions.Options.Options.Create
- Fix VexDecisionReachabilityIntegrationTests API drift (major rewrite)
- Fix VexSchemaValidationTests FluentAssertions method name
- Fix FixChainGateIntegrationTests ambiguous type references
- Fix AdvisoryAI test files required properties and namespace aliases
- Add stub types for CveMappingController (ICveSymbolMappingService)
- Fix VerdictBuilderService static context issue

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Policy/__Libraries/StellaOps.Policy.Predicates/FixChain/FixChainGateExtensions.cs

@@ -0,0 +1,89 @@
+// Licensed under AGPL-3.0-or-later. Copyright (C) 2026 StellaOps Contributors.
+// Sprint: SPRINT_20260110_012_008_POLICY
+// Task: FCG-003 - Policy Engine Integration
+
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using StellaOps.Policy.Gates;
+
+namespace StellaOps.Policy.Predicates.FixChain;
+
+/// <summary>
+/// Extension methods for registering FixChain gate services.
+/// </summary>
+public static class FixChainGateExtensions
+{
+    /// <summary>
+    /// Adds FixChain gate predicate services to the service collection.
+    /// </summary>
+    /// <param name="services">Service collection.</param>
+    /// <param name="configuration">Configuration section for options.</param>
+    /// <returns>Service collection for chaining.</returns>
+    public static IServiceCollection AddFixChainGate(
+        this IServiceCollection services,
+        IConfiguration? configuration = null)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        // Register options
+        if (configuration is not null)
+        {
+            services.AddOptions<FixChainGateOptions>()
+                .Bind(configuration.GetSection("Policy:Predicates:FixChainGate"))
+                .ValidateDataAnnotations()
+                .ValidateOnStart();
+        }
+        else
+        {
+            services.TryAddSingleton(new FixChainGateOptions());
+        }
+
+        // Register core services
+        services.TryAddSingleton<IFixChainGatePredicate, FixChainGatePredicate>();
+        services.TryAddSingleton<IFixChainGateBatchService, FixChainGateBatchService>();
+
+        // Register adapter for policy gate registry
+        services.TryAddSingleton<FixChainGateAdapter>();
+
+        return services;
+    }
+
+    /// <summary>
+    /// Adds FixChain gate predicate services with custom options.
+    /// </summary>
+    /// <param name="services">Service collection.</param>
+    /// <param name="configureOptions">Options configuration delegate.</param>
+    /// <returns>Service collection for chaining.</returns>
+    public static IServiceCollection AddFixChainGate(
+        this IServiceCollection services,
+        Action<FixChainGateOptions> configureOptions)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        ArgumentNullException.ThrowIfNull(configureOptions);
+
+        services.Configure(configureOptions);
+
+        // Register core services
+        services.TryAddSingleton<IFixChainGatePredicate, FixChainGatePredicate>();
+        services.TryAddSingleton<IFixChainGateBatchService, FixChainGateBatchService>();
+
+        // Register adapter for policy gate registry
+        services.TryAddSingleton<FixChainGateAdapter>();
+
+        return services;
+    }
+
+    /// <summary>
+    /// Registers the FixChain gate with the policy gate registry.
+    /// </summary>
+    /// <param name="registry">Policy gate registry.</param>
+    /// <returns>Registry for chaining.</returns>
+    public static IPolicyGateRegistry RegisterFixChainGate(this IPolicyGateRegistry registry)
+    {
+        ArgumentNullException.ThrowIfNull(registry);
+
+        registry.Register<FixChainGateAdapter>("fixChainRequired");
+        return registry;
+    }
+}