Restructure solution layout by module

src/__Libraries/StellaOps.Cryptography.DependencyInjection/CryptoProviderRegistryOptions.cs

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+using System.Collections.Generic;
+
+namespace StellaOps.Cryptography.DependencyInjection;
+
+/// <summary>
+/// Options controlling crypto provider registry ordering and selection.
+/// </summary>
+public sealed class CryptoProviderRegistryOptions
+{
+    /// <summary>
+    /// Ordered list of preferred provider names. Providers appearing here are consulted first.
+    /// </summary>
+    public IList<string> PreferredProviders { get; } = new List<string>();
+}

src/__Libraries/StellaOps.Cryptography.DependencyInjection/CryptoServiceCollectionExtensions.cs

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+using System;
+using System.Collections.Generic;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Options;
+using StellaOps.Cryptography;
+
+namespace StellaOps.Cryptography.DependencyInjection;
+
+/// <summary>
+/// Dependency injection helpers for registering StellaOps cryptography services.
+/// </summary>
+public static class CryptoServiceCollectionExtensions
+{
+    /// <summary>
+    /// Registers the default crypto provider and registry.
+    /// </summary>
+    /// <param name="services">Service collection.</param>
+    /// <param name="configureRegistry">Optional registry ordering configuration.</param>
+    /// <param name="configureProvider">Optional provider-level configuration (e.g. key registration).</param>
+    /// <returns>The service collection.</returns>
+    public static IServiceCollection AddStellaOpsCrypto(
+        this IServiceCollection services,
+        Action<CryptoProviderRegistryOptions>? configureRegistry = null,
+        Action<DefaultCryptoProvider>? configureProvider = null)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        if (configureRegistry is not null)
+        {
+            services.Configure(configureRegistry);
+        }
+
+        services.TryAddSingleton<DefaultCryptoProvider>(sp =>
+        {
+            var provider = new DefaultCryptoProvider();
+            configureProvider?.Invoke(provider);
+            return provider;
+        });
+
+        services.TryAddEnumerable(ServiceDescriptor.Singleton<ICryptoProvider, DefaultCryptoProvider>());
+
+#if STELLAOPS_CRYPTO_SODIUM
+        services.TryAddSingleton<LibsodiumCryptoProvider>();
+        services.TryAddEnumerable(ServiceDescriptor.Singleton<ICryptoProvider, LibsodiumCryptoProvider>());
+#endif
+
+        services.TryAddSingleton<ICryptoProviderRegistry>(sp =>
+        {
+            var providers = sp.GetServices<ICryptoProvider>();
+            var options = sp.GetService<IOptions<CryptoProviderRegistryOptions>>();
+            IEnumerable<string>? preferred = options?.Value?.PreferredProviders;
+            return new CryptoProviderRegistry(providers, preferred);
+        });
+
+        return services;
+    }
+}

src/__Libraries/StellaOps.Cryptography.DependencyInjection/StellaOps.Cryptography.DependencyInjection.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\StellaOps.Cryptography\StellaOps.Cryptography.csproj" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="10.0.0-rc.2.25502.107" />
+    <PackageReference Include="Microsoft.Extensions.Options" Version="10.0.0-rc.2.25502.107" />
+  </ItemGroup>
+</Project>