Restructure solution layout by module

src/Scanner/StellaOps.Scanner.Sbomer.BuildXPlugin/Cas/CasWriteResult.cs

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+namespace StellaOps.Scanner.Sbomer.BuildXPlugin.Cas;
+
+/// <summary>
+/// Result of persisting bytes into the local CAS.
+/// </summary>
+public sealed record CasWriteResult(string Algorithm, string Digest, string Path);

src/Scanner/StellaOps.Scanner.Sbomer.BuildXPlugin/Cas/LocalCasClient.cs

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+using System;
+using System.IO;
+using System.Security.Cryptography;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StellaOps.Scanner.Sbomer.BuildXPlugin.Cas;
+
+/// <summary>
+/// Minimal filesystem-backed CAS used when the BuildX generator runs inside CI.
+/// </summary>
+public sealed class LocalCasClient
+{
+    private readonly string rootDirectory;
+    private readonly string algorithm;
+
+    public LocalCasClient(LocalCasOptions options)
+    {
+        if (options is null)
+        {
+            throw new ArgumentNullException(nameof(options));
+        }
+
+        algorithm = options.Algorithm.ToLowerInvariant();
+        if (!string.Equals(algorithm, "sha256", StringComparison.OrdinalIgnoreCase))
+        {
+            throw new ArgumentException("Only the sha256 algorithm is supported.", nameof(options));
+        }
+
+        rootDirectory = Path.GetFullPath(options.RootDirectory);
+    }
+
+    public Task<CasWriteResult> VerifyWriteAsync(CancellationToken cancellationToken)
+    {
+        ReadOnlyMemory<byte> probe = "stellaops-buildx-probe"u8.ToArray();
+        return WriteAsync(probe, cancellationToken);
+    }
+
+    public async Task<CasWriteResult> WriteAsync(ReadOnlyMemory<byte> content, CancellationToken cancellationToken)
+    {
+        var digest = ComputeDigest(content.Span);
+        var path = BuildObjectPath(digest);
+
+        Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+
+        await using var stream = new FileStream(
+            path,
+            FileMode.Create,
+            FileAccess.Write,
+            FileShare.Read,
+            bufferSize: 16 * 1024,
+            FileOptions.Asynchronous | FileOptions.SequentialScan);
+
+        await stream.WriteAsync(content, cancellationToken).ConfigureAwait(false);
+        await stream.FlushAsync(cancellationToken).ConfigureAwait(false);
+
+        return new CasWriteResult(algorithm, digest, path);
+    }
+
+    private string BuildObjectPath(string digest)
+    {
+        // Layout: <root>/<algorithm>/<first two>/<rest>.bin
+        var prefix = digest.Substring(0, 2);
+        var suffix = digest[2..];
+        return Path.Combine(rootDirectory, algorithm, prefix, $"{suffix}.bin");
+    }
+
+    private static string ComputeDigest(ReadOnlySpan<byte> content)
+    {
+        Span<byte> buffer = stackalloc byte[32];
+        SHA256.HashData(content, buffer);
+        return Convert.ToHexString(buffer).ToLowerInvariant();
+    }
+}

src/Scanner/StellaOps.Scanner.Sbomer.BuildXPlugin/Cas/LocalCasOptions.cs

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+using System;
+
+namespace StellaOps.Scanner.Sbomer.BuildXPlugin.Cas;
+
+/// <summary>
+/// Configuration for the on-disk content-addressable store used during CI.
+/// </summary>
+public sealed record LocalCasOptions
+{
+    private string rootDirectory = string.Empty;
+    private string algorithm = "sha256";
+
+    public string RootDirectory
+    {
+        get => rootDirectory;
+        init
+        {
+            if (string.IsNullOrWhiteSpace(value))
+            {
+                throw new ArgumentException("Root directory must be provided.", nameof(value));
+            }
+
+            rootDirectory = value;
+        }
+    }
+
+    public string Algorithm
+    {
+        get => algorithm;
+        init
+        {
+            if (string.IsNullOrWhiteSpace(value))
+            {
+                throw new ArgumentException("Algorithm must be provided.", nameof(value));
+            }
+
+            algorithm = value;
+        }
+    }
+}