Add tests for SBOM generation determinism across multiple formats

- Created `StellaOps.TestKit.Tests` project for unit tests related to determinism.
- Implemented `DeterminismManifestTests` to validate deterministic output for canonical bytes and strings, file read/write operations, and error handling for invalid schema versions.
- Added `SbomDeterminismTests` to ensure identical inputs produce consistent SBOMs across SPDX 3.0.1 and CycloneDX 1.6/1.7 formats, including parallel execution tests.
- Updated project references in `StellaOps.Integration.Determinism` to include the new determinism testing library.

src/__Libraries/StellaOps.TestKit/Assertions/SnapshotAssert.cs

@@ -0,0 +1,114 @@
+using System.Text;
+using System.Text.Json;
+using Xunit;
+
+namespace StellaOps.TestKit.Assertions;
+
+/// <summary>
+/// Provides snapshot testing assertions for golden master testing.
+/// Snapshots are stored in the test project's `Snapshots/` directory.
+/// </summary>
+/// <remarks>
+/// Usage:
+/// <code>
+/// [Fact]
+/// public void TestSbomGeneration()
+/// {
+///     var sbom = GenerateSbom();
+///
+///     // Snapshot will be stored in Snapshots/TestSbomGeneration.json
+///     SnapshotAssert.MatchesSnapshot(sbom, snapshotName: "TestSbomGeneration");
+/// }
+/// </code>
+///
+/// To update snapshots (e.g., after intentional changes), set environment variable:
+/// UPDATE_SNAPSHOTS=1 dotnet test
+/// </remarks>
+public static class SnapshotAssert
+{
+    private static readonly bool UpdateSnapshotsMode =
+        Environment.GetEnvironmentVariable("UPDATE_SNAPSHOTS") == "1";
+
+    /// <summary>
+    /// Asserts that the value matches the stored snapshot. If UPDATE_SNAPSHOTS=1, updates the snapshot.
+    /// </summary>
+    /// <param name="value">The value to snapshot (will be JSON-serialized).</param>
+    /// <param name="snapshotName">The snapshot name (filename without extension).</param>
+    /// <param name="snapshotsDirectory">Optional directory for snapshots (default: "Snapshots" in test project).</param>
+    public static void MatchesSnapshot<T>(T value, string snapshotName, string? snapshotsDirectory = null)
+    {
+        snapshotsDirectory ??= Path.Combine(Directory.GetCurrentDirectory(), "Snapshots");
+        Directory.CreateDirectory(snapshotsDirectory);
+
+        string snapshotPath = Path.Combine(snapshotsDirectory, $"{snapshotName}.json");
+
+        // Serialize to pretty JSON for readability
+        string actualJson = JsonSerializer.Serialize(value, new JsonSerializerOptions
+        {
+            WriteIndented = true,
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+        });
+
+        if (UpdateSnapshotsMode)
+        {
+            // Update snapshot
+            File.WriteAllText(snapshotPath, actualJson, Encoding.UTF8);
+            return; // Don't assert in update mode
+        }
+
+        // Verify snapshot exists
+        Assert.True(File.Exists(snapshotPath),
+            $"Snapshot '{snapshotName}' not found at {snapshotPath}. Run with UPDATE_SNAPSHOTS=1 to create it.");
+
+        // Compare with stored snapshot
+        string expectedJson = File.ReadAllText(snapshotPath, Encoding.UTF8);
+
+        Assert.Equal(expectedJson, actualJson);
+    }
+
+    /// <summary>
+    /// Asserts that the text matches the stored snapshot.
+    /// </summary>
+    public static void MatchesTextSnapshot(string value, string snapshotName, string? snapshotsDirectory = null)
+    {
+        snapshotsDirectory ??= Path.Combine(Directory.GetCurrentDirectory(), "Snapshots");
+        Directory.CreateDirectory(snapshotsDirectory);
+
+        string snapshotPath = Path.Combine(snapshotsDirectory, $"{snapshotName}.txt");
+
+        if (UpdateSnapshotsMode)
+        {
+            File.WriteAllText(snapshotPath, value, Encoding.UTF8);
+            return;
+        }
+
+        Assert.True(File.Exists(snapshotPath),
+            $"Snapshot '{snapshotName}' not found at {snapshotPath}. Run with UPDATE_SNAPSHOTS=1 to create it.");
+
+        string expected = File.ReadAllText(snapshotPath, Encoding.UTF8);
+        Assert.Equal(expected, value);
+    }
+
+    /// <summary>
+    /// Asserts that binary data matches the stored snapshot.
+    /// </summary>
+    public static void MatchesBinarySnapshot(byte[] value, string snapshotName, string? snapshotsDirectory = null)
+    {
+        snapshotsDirectory ??= Path.Combine(Directory.GetCurrentDirectory(), "Snapshots");
+        Directory.CreateDirectory(snapshotsDirectory);
+
+        string snapshotPath = Path.Combine(snapshotsDirectory, $"{snapshotName}.bin");
+
+        if (UpdateSnapshotsMode)
+        {
+            File.WriteAllBytes(snapshotPath, value);
+            return;
+        }
+
+        Assert.True(File.Exists(snapshotPath),
+            $"Snapshot '{snapshotName}' not found at {snapshotPath}. Run with UPDATE_SNAPSHOTS=1 to create it.");
+
+        byte[] expected = File.ReadAllBytes(snapshotPath);
+        Assert.Equal(expected, value);
+    }
+}